can: dev: let can_get_echo_skb() return dlc of CAN frame

can_get_echo_skb() is usually called in the TX complete handler.
The stats->tx_packets and stats->tx_bytes should be updated there, too.
This patch simplifies to figure out the size of the sent CAN frame.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>