spi: fix spi.h kernel-doc warning

Fix kernel-doc warning in spi.h (copy/paste):

Warning(include/linux/spi/spi.h:365): No description found for parameter 'unprepare_transfer_hardware'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>