[PATCH] xtensa: use valid_signal()

xtensa should use valid_signal() instead of testing _NSIG directly like
everyone else.

Signed-off-by: Jesper Juhl <juhl-lkml@dif.dk>
Cc: Chris Zankel <chris@zankel.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>