b43: Refactor and update antenna diversity for A/G-PHY

-Make use of the b43_phy_set/mask/maskset helpers.
-Fix a few errors in the code.
-Make the code more readable.

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