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>