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

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_write'

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

Signed-off-by: Javier Martinez Canillas <[email protected]>
Signed-off-by: Mark Brown <[email protected]>
1 file changed