can: enable CAN FD for virtual CAN devices by default

CAN FD capable CAN interfaces can handle (classic) CAN 2.0 frames too.
New users usually fail at their first attempt to explore CAN FD on
virtual CAN interfaces due to the current CAN_MTU default.

Set the MTU to CANFD_MTU by default to reduce this confusion.
If someone *really* needs a 'classic CAN'-only device this can be set
with the 'ip' tool with e.g. 'ip link set vcan0 mtu 16' as before.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>