cfg80211: require add_virtual_intf to return new dev

cfg80211 used to do all its bookkeeping in
the notifier, but some new stuff will have
to use local variables so make the callback
return the netdev pointer.

Tested-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>