MIPS: Return ENOSYS from sys32_syscall on 64bit kernels like elsewhere.

When the o32 errno was changed to ENOSYS, we forgot to update the code
for 64bit kernels.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Acked-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>