mac80211: make ieee80211_find_sta per virtual interface

Since we have a TODO item to make all station
management dependent on virtual interfaces, I
figured I'd start with pushing such a change
to drivers before more drivers start using the
ieee80211_find_sta() API with a hw pointer and
cause us grief later on.

For now continue exporting the old API in form
of ieee80211_find_sta_by_hw(), but discourage
its use strongly.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>