cfg80211: fix sysfs registration race

My locking rework/race fixes caused a regression in the
registration, causing uevent notifications for wireless
devices before the device is really fully registered and
available in nl80211.

Fix this by moving the device_add() under rtnl and move
the rfkill to afterwards (it can't be under rtnl.)

Reported-and-tested-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>