mac80211: Add debugfs callbacks for station addition/removal

Provide drivers with hooks to create debugfs files when
a new station is added. This would help drivers to take
advantage of mac80211's station list infrastructure and not maintain
tedious station management code internally.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
[ifdef inline wrapper functions]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>