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>

include/linux/spi/spi.h

@@ -254,7 +254,7 @@ static inline void spi_unregister_driver(struct spi_driver *sdrv)
  *	driver is finished with this message, it must call
  *	spi_finalize_current_message() so the subsystem can issue the next
  *	transfer
- * @prepare_transfer_hardware: there are currently no more messages on the
+ * @unprepare_transfer_hardware: there are currently no more messages on the
  *	queue so the subsystem notifies the driver that it may relax the
  *	hardware by issuing this call
  *