b43: HT-PHY: implement controlling TX power control

Don't enable it until we have (almost?) whole TX power management
figured out. It's similar to the N-PHY, the difference is that we call a
"fix" *before* disabling power control.

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