mt7601u: remove warning when avg_rssi is zero

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: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>