mac80211: stop polling in disassociation

Stop connection monitor poll during disassociation.
This clears the polling flags and if a scan was
deferred it will be run.

Without this fix, if a scan was deferred due to
connection monitoring while disassociation happens,
this scan blocks further scan requests until interface
down/up which causes problems connecting to another AP.

Signed-off-by: David Spinadel <david.spinadel@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>