b43: HT-PHY: Move radio preparation into init function

Radio should be prepared only before initialization. We need this to be
able to call b43_radio_2059_init conditionally (in the future).
This also documents RF control register a bit.

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