mn10300: get rid of set_fs(USER_DS) in sigframe setup

It really has no business being there; short of a serious kernel bug
we should already have USER_DS at that point.  It shouldn't have been
done on x86 either...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>