b43: HT-PHY: implement CCA reset

It was just another similar-to-N-PHY and easy-to-track routine:
write32 0xb0601408 <- 0x00002057
 phy_read(0x0001) -> 0x0000
phy_write(0x0001) <- 0x4000
phy_write(0x0001) <- 0x0000
write32 0xb0601408 <- 0x00002055
(b43_phy_ht_force_rf_sequence was moved up unmodified)

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