ath9k: Cancel pending offchannel operations

This patch makes sure that pending roc/scan
operations are cancelled properly when a new
context is assigned/unassigned.

The flush_work() call to flush out any scheduled
channel context work is removed, instead, sc->mutex
is dropped to allow any pending work to get a chance
to complete by the channel scheduler. Also, increase
the timeout to allow a switch to an active GO.

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