mt76x0: mac: use sta ewma estimation for rssi tracking

Use shared mt76x02 utility routines for rssi tracking.
Moreover remove no longer used con_mon_lock spinlock
and following variable:
- ap_bssid
- bcn_freq_off
- bcn_phy_mode
- avg_rssi

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>