Document the AR9285/AR9485 LNA configuration information that's
stored in the ctl/ext RSSI field for chain 2.

Tested:

* AR9285, STA
Add a new capability flag to announce that the chip implements LNA mixing
for the RX path.

This is different to the div comb HAL flag, that says it actually
can use this for RX diversity (the "slow" diversity path implemented
but disabled in the AR9285 HAL code.)

Tested:

* AR9285, STA operation
Implement a bit of a hack to store the AR9285/AR9485 RX LNA configuration in
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
Handle/mark/nuke unused arguments.