rfkill: always init poll delayed work

The rfkill core didn't initialise the poll delayed work
because it assumed that polling was always done by specifying
the poll function. cfg80211, however, would like to start
polling only later, which is a valid use case and easy to
support, so change rfkill to always initialise the poll
delayed work and thus allow starting polling by calling the
rfkill_resume_polling() function after registration.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>