rt2x00: Reset antenna RSSI after switch

When the antenna configuration has changed we should reset
the antenna RSSI value. Otherwise the value will be influenced
by the previous configuration quality which in turn will affect
the antenna diversity.

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