mt76: mt7615: rework rx phy index handling

Overwriting the RMAC_CHFREQ register is not reliable enough, as the firmware
could potentially write it again.
Since there is no PHY index indication in the rx info, we need to use another
way:

If both PHYs are using different channels, find the PHY where chfreq matches
the register value.

The only corner case remaining is when both PHYs are using the same channel.
In that case, the per-packet noise value on the primary PHY will have
information belonging to the chains of the secondary PHY from the previous
received packet of that PHY. The secondary PHY will set noise to 0 for extra
chains.

Signed-off-by: Felix Fietkau <nbd@nbd.name>