b43: HT-PHY: switch radio to requested channel

Switching channel happens after specific SHM write to B43_SHM_SH_CHAN.
This is the way we found it in BCM4331 MMIO dumps. By comparing with
N-PHY code we noticed there is routing used for SYN and TX/RX.

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