can: gs_usb: gs_can_open(): allow loopback and listen only at the same time

There's no reason why loopback and listen only should not be allowed
at the same time. Replace the "else if" by "if" to reflect this in the
code.

Link: https://lore.kernel.org/all/20221019221016.1659260-2-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/usb/gs_usb.c

@@ -911,7 +911,8 @@ static int gs_can_open(struct net_device *netdev)
 	/* flags */
 	if (ctrlmode & CAN_CTRLMODE_LOOPBACK)
 		flags |= GS_CAN_MODE_LOOP_BACK;
-	else if (ctrlmode & CAN_CTRLMODE_LISTENONLY)
+
+	if (ctrlmode & CAN_CTRLMODE_LISTENONLY)
 		flags |= GS_CAN_MODE_LISTEN_ONLY;
 
 	/* Controller is not allowed to retry TX