nios2: fix syscall restart checks

sys_foo() returns -512 (aka -ERESTARTSYS) => do_signal() sees
512 in r2 and 1 in r1.

sys_foo() returns 512 => do_signal() sees 512 in r2 and 0 in r1.

The former is restart-worthy; the latter obviously isn't.

Fixes: b53e906d ("nios2: Signal handling support")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>