MIPS: Fix unreachable code issue

Fix the following warning (mips-randconfig):

arch/mips/include/asm/fpu.h:79:3: warning: fallthrough annotation in unreachable code [-Wimplicit-fallthrough]

Originally, the /* fallthrough */ comment was introduced by commit:

597ce1723e0f ("MIPS: Support for 64-bit FP with O32 binaries")

and it was wrongly replaced with fallthrough; by commit:

c9b029903466 ("MIPS: Use fallthrough for arch/mips")

As the original comment is actually useful, fix this issue by
removing unreachable fallthrough; statement and place the original
/* fallthrough */ comment back.

Fixes: c9b029903466 ("MIPS: Use fallthrough for arch/mips")
Reported-by: kernel test robot <[e-mail]>
Link: https://lore.kernel.org/lkml/[e-mail]/
Signed-off-by: Gustavo A. R. Silva <[e-mail]>

arch/mips/include/asm/fpu.h

···
 		/* we only have a 32-bit FPU */
 		return SIGFPE;
 #endif
-		fallthrough;
+		/* fallthrough */
 	case FPU_32BIT:
 		if (cpu_has_fre) {
 			/* clear FRE */