frv: fix check on unsigned in do_signal()

syscallno is unsigned

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>