ath9k_htc: fix signal strength handling issues

The ath9k commit 2ef16755


(ath9k: fix signal strength reporting issues) fixed an issue where the
reported per-frame signal strength reported to mac80211 was being
overwritten with an internal average. The same issue is also present
in ath9k_htc.
In addition to preventing the driver from overwriting the value, this
commit also ensures that the internal average (which is used for ANI)
only tracks beacons of the AP that we're connected to.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@vger.kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>