Do not start wpa_radio work during externally triggered scan

Do not start wpa_radio work during externally triggered scan

If an external program triggers a scan, wpa_supplicant does not have a
wpa_radio work item for this operation to protect against other
offchannel operations. This can result in operations failing, so try to
avoid damage by not starting any new wpa_radio work items during a scan
that was started by another process.

Signed-hostap: Jouni Malinen <j@w1.fi>