ath9k: Fix offchannel flush timeout

An offchannel operation also needs to have
a flush timeout that doesn't exceed the NoA
absence duration of a GO context, so use
channel_switch_time. The first offchannel
operation is set a flush timeout of 10ms since
channel_switch_time will be zero.

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