b43: HT-PHY: use separated function for forcing RF sequence

Comparison of the HT and N code has shown similarities in the ops
performed after b43_mac_phy_clock_set. That way we understood what is
happening in the HT-PHY code.

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