mac80211: add vif debugfs driver callbacks

Add debugfs driver callbacks so drivers can add
debugfs entries for interfaces. Note that they
_must_ remove the entries again as add/remove in
the driver doesn't correspond to add/remove in
debugfs; the former is up/down while the latter
is netdev create/destroy.

Signed-off-by: Alexander Bondar <alexander.bondar@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>