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

include/linux/spi/spi.h

@@ -394,6 +394,7 @@ static inline void spi_unregister_driver(struct spi_driver *sdrv)
  *                   for example doing DMA mapping.  Called from threaded
  *                   context.
  * @transfer_one: transfer a single spi_transfer.
+ *
  *                  - return 0 if the transfer is finished,
  *                  - return 1 if the transfer is still in progress. When
  *                    the driver is finished with this transfer it must