[ath_hal] implement shared PA handling checks, based on ath9k.

These are apparently conditional on there being a shared PA/LNA, which
at least on AR9462/QCA9535 devices I have isn't a thing.

I'm .. not yet sure which devices it /is/ a thing, so I'll come back
to that.

Tested:

• QCA9565 STA + bluetooth

Obtained from: Linux ath9k