SPI broken for STM32L1

[Mani-Sadhasivam]

Describe the bug
With the current Zephyr master, SPI is not working as expected on STM32L1 based 96boards Wistrio board. Some of the bytes of the received data seems to be dropping.

To Reproduce
Reproducible with the LoRa PR #18998 which uses SPI for communicating to the Radio.

The issue seems to be with this commit: 2ce8fa1
To be more precise, setting xfer_16 flag manually to false fixes the issue, which means the problem is with sending/receiving 16bits of data.

[Mani-Sadhasivam]

@psidhu Since you are the actual author of the commit, could you please look into this?

[erwango]

@Mani-Sadhasivam, thanks for reporting. This is partly my bad, not enough tests have been made for this behavior to be made used by default.
@psidhu, can you push a change to set this behavior available only under specific Kconfig flag (default n)?

[psidhu]

@erwango Sure, though it may take me a small bit of time to get to.