b43: HT-PHY: implement switching analog

Turning it on is always done between reading PHY version and radio
version, so it was easy to find it in MMIO dumps from ndiswrapper.
Turning off is done by writing different values to the same registers.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>