rt2x00: Implement set_tim callback for all drivers

Implement set_tim callback for all rt2x00 drivers, this makes the
device wake up powersaving stations properly while in AP mode.

The only way to update the beacon is by simply calling mac80211 and
requesting the new beacon. This means the set_tim() event is mostly the
same as a beacon_done() event which was already defined in rt2x00lib.

Signed-off-by: Stefan Steuerwald <salsasepp@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>