b43: Implement antenna diversity support for LP-PHY

The A/G-PHY changes are fallout fixes from the enum change,
which in turn allows the LP-PHY code to be much simpler.
The antenna_to_phyctl change is a fix for a potential
existing bug that this patch may otherwise trigger.

Signed-off-by: Gábor Stefanik <netrolller.3d@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>