ath9k: Assign offchannel duration properly

In multi-channel mode, an offchannel request will
be deferred if both contexts are active. The duration
of the offchannel operation is calculated but is
not stored in the scheduler state. Fix this.

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