rt2x00: Introduce sta_add/remove callbacks

This implements a basic sta_add and sta_remove callback. Introduce a new
structure rt2x00_sta and ask mac80211 to allocate it as private part of
its ieee80211_sta. rt2x00_sta only contains the WCID for now.

The sta_add callback allows the driver to assign a WCID to a station
that is currently being added. The same wcid is also passed to the
sta_remove callback one mac80211 removes this STA.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>