spi: Add gfp parameter to kernel-doc to fix build warning

The spi_split_transfers_maxsize() gfp parameter is missing in the
function kernel-doc so building gives the following warning:

.//drivers/spi/spi.c:2359: warning: No description found for parameter 'gfp'
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -2319,6 +2319,7 @@ static int __spi_split_transfer_maxsize(struct spi_master *master,
  * @master:    the @spi_master for this transfer
  * @msg:   the @spi_message to transform
  * @maxsize:  the maximum when to apply this
+ * @gfp: GFP allocation flags
  *
  * Return: status of transformation
  */