ath9k: Fix issue with MCS15

On some boards which are based on AR9300, AR9580 or
AR9550, MCS15 usage is problematic.

This is because these boards use a "frequency doubler",
which doubles the refclk to get better EVM, but causes
spurs. Handle this properly in the driver to recover
throughput.

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