ath9k_htc: Fix fast channel change

When returning to the operating channel, a full HW
reset has to be done instead of a fast channel change.
Since sw_scan_complete() is called after the config() call for the
home channel, we end up doing a FCC. Fix this issue by checking
the OFFCHANNEL flag to determine FCC.

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