Until there's a full MCI implementation - just implement a placeholder

Until there's a full MCI implementation - just implement a placeholder
MCI bluetooth coexistence method for WB222.

The rest of MCI requires a bunch more work, including adding a DMA buffer
for the MCI hardware to bounce messages in/out of and handling MCI
interrupts.  But the more important part here is telling the HAL
the btcoex is enabled and MCI is in use so it configures the correct
initial bluetooth parameters in the wireless NIC and configures
things like bluetooth traffic weights and such.

So, this at least gets the HAL to do some of the right things in
configuring the inital bluetooth coexistence stuff, but doesn't
actually do full btcoex.  That'll take.. some effort.

Tested:

* AR9462 (WB222), STA mode