spi: fix duplicated word in <linux/spi/spi.h>

Change doubled word "as" to "as a".

Change "Return: Return:" in kernel-doc notation to have only one
"Return:".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: linux-spi@vger.kernel.org
Link: https://lore.kernel.org/r/40354d64-be71-3952-a980-63a76a278145@infradead.org


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