Fix the following fallthrough warning (nds32-randconfig with GCC):

include/math-emu/op-common.h:332:8: warning: this statement may fall through [-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>