alpha: cpumask_of_node() should handle -1 as a node

CC: Richard Henderson <rth@twiddle.net>
CC: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Matt Turner <mattst88@gmail.com>
CC: linux-alpha@vger.kernel.org
CC: Rusty Russell <rusty@rustcorp.com.au>
CC: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Anton Blanchard <anton@samba.org>