ath9k: Avoid HW opmode overridden on monitor mode changes

The HW opmode is blindly set to monitor type on monitor mode
change notification. This overrides the opmode when one of the
interfaces is still running as non-monitor iftype. So the monitoring
information needs to be maintained seperately.

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>