ath9k: Clear offchannel duration properly

Clearing the offchannel duration value in the
scheduler unconditionally breaks NoA when
multiple contexts are active and an offchannel
request is deferred, for example, in a scan run.

Fix this by clearing the duration only if there
is no pending offchannel request.

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