as an association ID is set any scan is supposed to be a background scan.
This implies that the firmware will switch back to the associated channel
after a certain threshold, though, we are not notified about that. We
currently catch this case by a timer which will reset the firmware after
a 'scan timeout', though, upper layers are not notified about that and
will simply hang until manual intervention. Fix this by resetting the
firmware's knowledge about any association on RUN -> ASSOC and
!INIT -> SCAN transitions.
Tested by: Zhihao Yuan <lichray at gmail.com>
MFC after: 1 week
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