rt2x00: Disable link tuning in AP mode

Since the link tuning is based on average RSSI values taken from all received
frames it doesn't make sense to enable it in AP mode where every associated
station provides independent RSSI values. Furthermore the legacy drivers
don't enable link tuning in AP mode as well.

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>