spi: fix kernel-doc warnings about missing return desc in spi.c

When building docs with make htmldocs, warnings about not having
a description for the return value are reported, i.e:

warning: No description found for return value of 'spi_register_driver'

Fix these by following the kernel-doc conventions explained in
Documentation/kernel-doc-nano-HOWTO.txt.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>