Allow updating single fields of InterrupterRegisterSet

[Demindiro]

This is necessary to be able to update ERDP without confusing the controller (in QEMU, at least).

For example, this code works:

		reg.erdp
			.update_volatile(|c| c.set_event_ring_dequeue_pointer(phys));

xhci: reset while running!
xhci_ep_nuke_xfers(1, 1)
xhci_ep_nuke_xfers(1, 3)
xhci: event ring[0]:1129000 [32]
xhci_process_commands()
xhci_process_commands()
xhci: input context at 122a000

But this doesn't (which is what the old API effectively did):

		reg.erdp
			.update_volatile(|c| c.set_event_ring_dequeue_pointer(phys));
		reg.erstba.update_volatile(|c| c.set(self.buf.as_phys()));

xhci: reset while running!
xhci_ep_nuke_xfers(1, 1)
xhci_ep_nuke_xfers(1, 3)
xhci: event ring[0]:112a000 [32]
xhci: event ring[0]:112a000 [32]
xhci_process_commands()
xhci: ER 0 full, drop event

Since this is an API break I also took the liberty to rename InterruptRegisterSet to the technically correct InterrupterRegisterSet. I added a type alias to reduce confusion.

Fixes #115

[toku-sa-n]

Thank you for the PR. I'm going to go on a trip for three days from tomorrow. I'll check this PR after that.

[toku-sa-n]

I appreciate your patience. I plan to release two versions:

• 0.8.7: Rename the struct and add a type alias (which is marked as deprecated).
• 0.9.0: Remove the type alias (and fix the structure of InterrupterRegisterSet which I haven't checked yet).

Could you split the two things and send another PR for the renaming?

[toku-sa-n]

Thank you for #143. Could you rebase this PR?

[toku-sa-n]

Thank you. The code looks good. I'll check this PR with my OS later. Meanwhile, could you apply rustfmt?

[toku-sa-n]

Thank you. There are some Clippy warnings: https://github.com/rust-osdev/xhci/runs/7940871377?check_suite_focus=true
Could you fix them?

[toku-sa-n]

Thanks for the fix. The code looks good to me. I'm still trying to try this PR with my OS, but (un)fortunately, this PR revealed a defect in my OS. I'm struggling to fix it, but I'll merge this PR anyway if I can't fix it shortly.
Just a confirmation, but does your OS work well with this PR?

---

By the way, I've figured out why the old API confused QEMU. The description of ERSTBA which is on page 428 of xHCI specification 1.2 says:

Writing this register sets the Event Ring State Machine:EREP Advancement to the Start state.

Thus, writing to any register in an interrupter register set made an event ring reset.

[Demindiro]

Just a confirmation, but does your OS work well with this PR?

It does work with USB keyboard, though I haven't tried other devices yet.

[toku-sa-n]

Thank you. Sorry for the inconvenience, but please wait one or two days.

[toku-sa-n]

I confirmed this PR with my OS, and it works! Thank you for your contributions and patience!
I'm sorry, but I left a comment. After resolving it, I'll release 0.9.0.

[toku-sa-n]

Thanks! I'll release 0.9.0 soon.

[toku-sa-n]

Published as 0.9.0.