the RX antenna field.
The AR9285/AR9485 use an LNA mixer to determine how to combine the signals
from the two antennas. This is encoded in the RSSI fields (ctl/ext) for
chain 2. So, let's use that here.
This maps RX antennas 0->3 to the RX mixer configuration used to
receive a frame. There's more that can be done but this is good enough
to diagnose if the hardware is doing "odd" things like trying to
receive frames on LNA2 (ie, antenna 2 or "alt" antenna) when there's
only one antenna connected.
Tested:
* AR9285, STA mode
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