docs: spi: spi.h: fix a doc building warning

We need to add a blank line to avoid this warning:

	./include/linux/spi/spi.h:401: WARNING: Unexpected indentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1c701b3ac903dc0bc304dca958fbdee53bd38dc3.1586881715.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>