[PATCH] spi: Add documentation for last_cs_index_mask

[Ruben Wauters <rubenru09@xxxxxxx>]

Documentation was missing for last_cs_index_mask, this was shown by
a warning in make htmldocs.

I have based the documentation off the docs for cs_index_mask, given
that they should be similar.

This is my first patch, so I hope I did everything right, please let
me know if I need to change something, and I shall endevour to do it
properly.

Signed-off-by: Ruben Wauters <rubenru09@xxxxxxx>
---
 include/linux/spi/spi.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index c459809efee4..ed941e65791a 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -453,6 +453,8 @@ 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: the last active chip select bit masks in the chip select
+ *                      array
  * @xfer_completion: used by core transfer_one_message()
  * @busy: message pump is busy
  * @running: message pump is running
-- 
2.40.1