[MIPS] Fix sigset_t endianess swapping issues in 32-bit compat code.

access_ok in compat-signal.h is wrong (checking destination instead of
source) and redundant (already checked before calling this function).

Also sf_mask in struct sigframe32 should be compat_sigset_t type.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>