fix(rtc): 用 SecondFraction/SubSeconds 计算 tv_usec，并在 shift pending 时跳过

[wdfk-prog]

Use the generic SecondFraction/SubSeconds formula when SSR/PRER are available. If SHPF is present and a shift is pending, keep tv_usec at 0. Platforms without SSR/PRER (e.g. F1) default to 0.

拉取/合并请求描述：(PR description)

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为什么提交这份PR (why to submit this PR)

修正 RTC gettimeofday 微秒字段计算逻辑：

• 当 MCU 支持 SSR/PRER（RTC_SSR_SS & RTC_PRER_PREDIV_S）时，使用 HAL 返回的 SecondFraction/SubSeconds 按通用公式计算 tv_usec
• 若平台提供 RTC_ISR_SHPF，当 shift 操作 pending（SHPF=1）时跳过换算，保持 tv_usec=0
• 对不支持 SSR/PRER 的系列（如 STM32F1）默认 tv_usec=0

你的解决方案是什么 (what is your solution)

请提供验证的bsp和config (provide the config and bsp)

• BSP:

• .config:

• action:

]

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• 代码是高质量的 Code in this PR is of high quality
• 已经使用formatting 等源码格式化工具确保格式符合RT-Thread代码规范 This PR complies with RT-Thread code specification
• 如果是新增bsp, 已经添加ci检查到.github/ALL_BSP_COMPILE.json 详细请参考链接BSP自查

[github-actions]

👋 感谢您对 RT-Thread 的贡献！Thank you for your contribution to RT-Thread!

为确保代码符合 RT-Thread 的编码规范，请在你的仓库中执行以下步骤运行代码格式化工作流（如果格式化CI运行失败）。
To ensure your code complies with RT-Thread's coding style, please run the code formatting workflow by following the steps below (If the formatting of CI fails to run).

---

🛠 操作步骤 | Steps

1. 前往 Actions 页面 | Go to the Actions page
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• 设置需排除的文件/目录（目录请以"/"结尾）
  Set files/directories to exclude (directories should end with "/")
• 将目标分支设置为 \ Set the target branch to：stm32_rtc
• 设置PR number为 \ Set the PR number to：11167

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   The formatted code will be automatically pushed to your branch.

完成后，提交将自动更新至 stm32_rtc 分支，关联的 Pull Request 也会同步更新。
Once completed, commits will be pushed to the stm32_rtc branch automatically, and the related Pull Request will be updated.

如有问题欢迎联系我们，再次感谢您的贡献！💐
If you have any questions, feel free to reach out. Thanks again for your contribution!

[github-actions]

📌 Code Review Assignment

🏷️ Tag: bsp_stm32

Reviewers: @Liang1795 @hamburger-os @wdfk-prog

Changed Files (Click to expand)

• bsp/stm32/libraries/HAL_Drivers/drivers/drv_rtc.c

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📊 Current Review Status (Last Updated: 2026-02-03 14:48 CST)

• ⌛ @Liang1795 Pending Review
• ⌛ @hamburger-os Pending Review
• ⌛ @wdfk-prog Pending Review

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1. 维护者可以通过单击此处来刷新审查状态: 🔄 刷新状态
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   Comment LGTM/lgtm after confirming approval

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