spi: spi.h: add missing kernel-doc for @last_cs_index_mask

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>

include/linux/spi/spi.h

@@ -453,6 +453,7 @@ extern struct spi_device *spi_new_ancillary_device(struct spi_device *spi, u8 ch
  * @last_cs_mode_high: was (mode & SPI_CS_HIGH) true on the last call to set_cs.
  * @last_cs: the last chip_select that is recorded by set_cs, -1 on non chip
  *           selected
+ * @last_cs_index_mask: bit mask the last chip selects that were used
  * @xfer_completion: used by core transfer_one_message()
  * @busy: message pump is busy
  * @running: message pump is running