rt2x00: Don't switch to antenna with low rssi

When rssi_a > rssi_b is true and the current antenna
was already antenna A, then rt2x00 incorrectly jumped
to antenna B.

Also don't configure the antenna when there has been
no change in the antenna setup.

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