can: don't use deprecated license identifiers

The "GPL-2.0" license identifier changed to "GPL-2.0-only" in SPDX v3.0.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>