ovs-vsctl: Verify VLAN bridge controllers in cmd_get_controller().

ovs-vsctl: Verify VLAN bridge controllers in cmd_get_controller().

A VLAN bridge uses its parent's controllers, so checking the controller
should verify the parent's set of controllers.

The change to verify_controllers() isn't necessary; it just deletes
the check for a null 'bridge' because verify_controllers() can no
longer be called with a null 'bridge'.

This fixes a bug, but it is unlikely to ever have caused a real problem for
users.

Found by inspection.

Signed-off-by: Ben Pfaff <blp@nicira.com>