When doing a scan while being associated it is possible that the scan
is deferred for the time it takes to flush the TX queue. This work being
done the scan then is continued, but only if it is marked to do so. As
the 'ifconfig scan' request is meant to be used after the interface is
brought up, request a background scan by default. This behaviour is
already documented in manual page.

This fixes on possible case where 'ifconfig scan' hangs infinitely.

MFC after: 1 month