kernel-doc complains about last_cs_index_mask not described, so add its
description.

spi.h:778: warning: Function parameter or struct member 'last_cs_index_mask' not described in 'spi_controller'

Fixes: 4d8ff6b0 ("spi: Add multi-cs memories support in SPI core")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://lore.kernel.org/r/20240328223340.17159-1-rdunlap@infradead.orgSigned-off-by: Mark Brown <broonie@kernel.org>