can: gs_usb: gs_usb_probe(): introduce udev and make use of it

Introduce the variable udev in the gs_usb_probe() function to hold a
pointer to the struct usb_device. This avoids recalculating the value
several times in this function.

Link: https://lore.kernel.org/all/20220309124132.291861-13-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>