sh: Fix up spurious syscall restarting.

The T-bit manipulation for syscall error checking had the side effect of
spuriously returning ERESTART* errno values over EINTR. So, we simplify
the error checking a bit and leave the T-bit alone.

Reported-by: Kaz Kojima <kkojima@rr.iij4u.or.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>