can: gs_usb: Added support for the GS_USB CAN devices

The Geschwister Schneider Family of devices are galvanically isolated USB2.0 to
CAN2.0A/B adapters. Currently two form factors are available, a tethered dongle
in a rugged enclosure, and mini-pci-e card.

Signed-off-by: Maximilian Schneider <max@schneidersoft.net>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>