spi: Fix htmldocs build error due struct spi_replaced_transfers

The kernel-doc has to be just before the structure definition but the one
for struct spi_replaced_transfers was before a structure declaration and
that confuses kernel-doc which complains with the following build error:

.//include/linux/spi/spi.h:933: error: Cannot parse struct or union!

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