b43: HT-PHY: implement radio ops

Every PHY has some specific bit used for reading radio regs. Analyze of
MMIO dumps from BCM4331 and ndiswrapper has shown it is 0x200 for HT.
 radio_read(0x037f) -> 0x0073
radio_write(0x017f) <- 0x0072

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