[MIPS] Fix bitops for MIPS32/MIPS64 CPUs.

    
With recent rewrite for generic bitops, fls() for 32bit kernel with
MIPS64_CPU is broken.  Also, ffs(), fls() should be defined the same
way as the libc and compiler built-in routines (returns int instead of
unsigned long).
    
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>