It turned out that we can run calibration without already received
frames with RSSI data. In such case just don't update AGC register
and don't print the warning.

Fixes: b305a6ab ("mt7601u: use EWMA to calculate avg_rssi")
Reported-by: NAdam Borowski <kilobyte@angband.pl>
Signed-off-by: NStanislaw Gruszka <sgruszka@redhat.com>
Acked-by: NJakub Kicinski <kubakici@wp.pl>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>