sh: Fix up unsigned syscall_nr in SH-5 pt_regs.

syscall_nr is presently defined as unsigned in the SH-5 pt_regs,
while the syscall restarting code wants it to be signed. Fix this
up, and bring it in line with the other SH parts.

Reported-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>