[ath_hal] add AR9462 (jupiter) RX gain / XLNA programming.

This seems to make 5G work better.

It doesn't fix powersave handling though, that still sees the PHY get
stuck during initial calibration and everything goes pear shaped.
I'll look into that later.

Tested:

• QCAFN222 NIC, STA mode, 5GHz

Obtained from: Linux ath9k