Do not start another connect work while one is pending

Do not start another connect work while one is pending

It was possible for the connect or sme-connect radio work to get
re-scheduled while an earlier request was still pending, e.g.,
select_network is issued at the moment a scan radio work is in progress
and the old scan results are recent enough for starting the connection.
This could result in unexpected attempt to re-associate immediately
after completing the first connection.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>