can: gs_usb: support up to 3 channels per device

Some STM32G3 chips support up to 3 CAN-FD channels, increase number of
supported channels in this driver to 3 accordingly.

Link: https://lore.kernel.org/all/20220309124132.291861-14-mkl@pengutronix.de
Suggested-by: Ryan Edwards <ryan.edwards@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>