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:

	597ce172 ("MIPS: Support for 64-bit FP with O32 binaries")

and it was wrongly replaced with fallthrough; by commit:

	c9b02990 ("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: c9b02990 ("MIPS: Use fallthrough for arch/mips")
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>