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.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>