rt2x00: fix beacon reset on rt2800

When an interface is removed the according beacon entry should be reset.
The current approach to only clear the first word is not enough to stop
the device from sending out the beacon, hence resulting in beacons being
sent out for already removed interfaces.

Fix this by invalidating the entire TXWI in front of the beacon instead
of only the first word.

Also clear all beacons during startup in the same way.

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>