ath9k: Set offchannel state properly

When switching offchannel, BSS related information
in the HW has to be reset to default values. Add a routine
to do this.

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