spi: Document max_transfer_size

Fix kernel-doc warning for missing struct field notation.

..//include/linux/spi/spi.h:540: warning: No description found for parameter 'max_transfer_size'

[Meaningful subject -- broonie]
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

include/linux/spi/spi.h

@@ -303,6 +303,8 @@ static inline void spi_unregister_driver(struct spi_driver *sdrv)
  * @min_speed_hz: Lowest supported transfer speed
  * @max_speed_hz: Highest supported transfer speed
  * @flags: other constraints relevant to this driver
+ * @max_transfer_size: function that returns the max transfer size for
+ *	a &spi_device; may be %NULL, so the default %SIZE_MAX will be used.
  * @bus_lock_spinlock: spinlock for SPI bus locking
  * @bus_lock_mutex: mutex for SPI bus locking
  * @bus_lock_flag: indicates that the SPI bus is locked for exclusive use