rfkill: Use subsys_initcall

We must use subsys_initcall, because we must initialize before a
driver calls rfkill_register().

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>