libnetwork/drivers/remote: rollbackEndpoint.Interface: return concrete type

Interface-matching should generally happen on the receiver side, and this
function was only used in a single location, and passed as argument to
Driver.CreateEndpoint, which already matches the interface by accepting
a driverapi.InterfaceInfo.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libnetwork/drivers/remote/driver_test.go

@@ -546,7 +546,7 @@ func TestMissingValues(t *testing.T) {
 
 type rollbackEndpoint struct{}
 
-func (r *rollbackEndpoint) Interface() driverapi.InterfaceInfo {
+func (r *rollbackEndpoint) Interface() *rollbackEndpoint {
 	return r
 }