can: gs_usb: gs_usb_probe(): use descriptors of current altsetting

Make sure to always use the descriptors of the current alternate setting
to avoid future issues when accessing fields that may differ between
settings.

Signed-off-by: Johan Hovold <johan@kernel.org>
Fixes: d08e973a

 ("can: gs_usb: Added support for the GS_USB CAN devices")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>