Let VMAC interface flags (up/down status) follow base interface.

When using a VMAC interface, this fix will reflect the base interface flags, i.e. up/down status, to the VMAC interface. This is useful when using sync groups (in combination with VMAC) and a link for one of the members in the MASTER sync group goes down. Before this fix, this member will not detect the link fault, due to that the VMAC interface always is UP regardless of the actual status of the base interface, and the sync group will continue to
be MASTER as if nothing has happend. This fix will however reflect the status of the base interface onto the VMAC interface, so if the link goes down the member will transit to FAULT state, which will make the sync group transit to BACKUP state.