m68knommu: fix syscall tracing stuck process

The return path from an exception was checking too many bits in the
thread_info->flags, and getting stuck calling do_signal(). There was
no work to do, we should only be checking the low 8 bits (as per comments
and definitions in arch/m68k/include/asm/thread_info.h).

This fixes the stuck process problem when using strace.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>