ath9k_hw: Cleanup FastChannelChange

The logic to determine whether to use FCC is a bit convoluted.
Use a small helper function to decide whether FCC is to be
used.

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