[RFKILL]: Fix check for correct rfkill allocation

coverity has spotted a bug in rfkill.c (bug id #1627),
in rfkill_allocate() NULL was returns if the kzalloc() works,
and deref the NULL pointer if it fails,

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>