rt2x00: Remove set_tim callback from PCI drivers

Using the set_tim callback without managing the DTIM count and the
broad- and multicast buffering in hw, fw or the driver results in wrong
DTIM count values being sent out in beacons. Since all PCI drivers
fetch new beacons periodically and hence get an updated TIM we can just
remove the set_tim callback from these.

The rt2x00 USB drivers don't update the beacon periodically and thus
rely on the set_tim callback to get a correct TIM for beacon
transmission. USB devices still suffer from the DTIM count being wrong
under some circumstances but removing the set_tim callback from these
would cause more harm then good.

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>