mmc: jz4740: Fix fall-through warning for Clang

Fix the following fallthrough warning (mips-randconfig with Clang):

drivers/mmc/host/jz4740_mmc.c:792:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/60edca25.k00ut905IFBjPyt5%25lkp@intel.com/


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>