mt76: improve gain adjustment in noisy environments

When switching between low gain (high RSSI) and high gain settings, it
can take a few seconds to adjust to the current environment.
This can lead to short periods of time with extreme packet loss.

When switching from low_gain=1 to low_gain=2, start with the same gain
adjustment value instead of the lowest to avoid spikes of huge numbers
of false CCA events

Also avoid resetting adjustment values on switching between low_gain
values 0 and 1, since it affects only the upper limit of vga adjustment

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>