wl1251: implement connection quality monitoring

Implement connection quality monitoring similar to the wl1271 driver.
It triggers ieee80211_cqm_rssi_notify with the corresponding event when
RSSI drops blow RSSI threshold or rises again above the RSSI threshold.
It should be noted that wl1251 doesn't support RSSI hysteresis, instead it
uses RSSI averageing and delays events until a certain count of frames
proved RSSI change.

Signed-off-by: David Gnedt <david.gnedt@davizone.at>
Acked-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>