spi: microchip-core: Refactor FIFO read and write handlers

Make both handlers to be shorter and easier to understand.
While at it, unify their style.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Prajna Rajendra Kumar <prajna.rajendrakumar@microchip.com>
Link: https://patch.msgid.link/20251127190031.2998705-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>