b43: LP-PHY: Implement channel switching for rev2+/B2063 radio

Rev.2+/B2063 will now hopefully show some signs of life, though
it won't work at full performance, as calibration is still missing.

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