spi: slave: Fix missing break in switch

Apparently, this code does not actually fall through to the next case
because the machine restarts before it has a chance. However, for the
sake of maintenance and readability, we better add the missing break
statement.

Addresses-Coverity-ID: 1437892 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Mark Brown <broonie@kernel.org>