ath9k: Initialize baseband for DFS channels

Certain baseband registers require different values
to be programmed when operating in a DFS channel to
ensure that radar detection works correctly. This
is required for AR9300, AR9340 and AR9580.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>