math-emu: Fix fall-through warning

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: Nkernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/60edca25.k00ut905IFBjPyt5%25lkp@intel.com/Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>

include/math-emu/op-common.h

@@ -338,7 +338,7 @@ do {									     \
 	FP_SET_EXCEPTION(FP_EX_INVALID | FP_EX_INVALID_ISI);		     \
 	break;								     \
       }									     \
-    /* FALLTHRU */							     \
+    fallthrough;							     \
 									     \
   case _FP_CLS_COMBINE(FP_CLS_INF,FP_CLS_NORMAL):			     \
   case _FP_CLS_COMBINE(FP_CLS_INF,FP_CLS_ZERO):				     \