cfg80211: fix rfkill locking problem

rfkill currently requires a global lock within the
rfkill_register() function, and holds that lock over
calls to the set_block() methods. This means that we
cannot hold a lock around rfkill_register() that we
also require in set_block(), directly or indirectly.
Fix cfg80211 to register rfkill outside the block
locked by its global lock. Much of what cfg80211 does
in the locked block doesn't need to be locked anyway.

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