spi: correct kernel-doc inconsistency

Silence documentation build warnings by correcting kernel-doc comment
for spi_transfer struct.

Signed-off-by: Colton Lewis <colton.w.lewis@protonmail.com>
Link: https://lore.kernel.org/r/20200725050242.279548-1-colton.w.lewis@protonmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>