can: gs_usb: remove gs_can::iface

The iface field of struct gs_can is only used to retrieve the
usb_device which is already available in gs_can::udev.

Replace each occurrence of interface_to_usbdev(dev->iface) with
dev->udev. This done, remove gs_can::iface.

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://lore.kernel.org/all/20221208081142.16936-3-mailhol.vincent@wanadoo.fr
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>