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>