cfg80211: always notify userspace of new wireless netdevs

This change alters the semantics of NL80211_CMD_NEW_INTERFACE events
by always sending this event whenever a new net_device object
associated with a wdev is registered.  Prior to this change, this event
was only sent as a result of NL80211_CMD_NEW_INTERFACE command sent
from userspace.  This allows userspace to reliably detect new wireless
interfaces (e.g. due to hardware hot-plug events, etc).

For wdevs created without an associated net_device object (e.g.
NL80211_IFTYPE_P2P_DEVICE), the NL80211_CMD_NEW_INTERFACE event is
still generated inside the relevant nl80211 command handler.

Signed-off-by: Denis Kenzior <denkenz@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>