can: slcan: Replace sizeof struct can_frame with CAN_MTU

Use CAN_MTU macro instead of sizeof(struct can_frame) just like the
other drivers do.

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