spi: AMD SPI controller driver bug fix and cleanups

Merge series from Cristian Ciocaltea <cristian.ciocaltea@collabora.com>:

This patch series addresses an issue in the spi-amd driver and, while
there, performs some additional cleanups, like simplifying the error
handling in the probe function and removing an unused struct member.

For improving code readability, it also adds some kernel-doc comments.