b43: HT-PHY: implement killing radio

Closed drivers kill radio right after reading radio version and MACCTL,
so it was easy to find related PHY ops:
 phy_read(0x0810) -> 0x0000
phy_write(0x0810) <- 0x0000
To find out the mask of above OP, MMIO hack was used to fake read val:
 phy_read(0x0810) -> 0xffff
phy_write(0x0810) <- 0x0000

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