We can avoid the HW access in TX cleanup path for retrieving the DLC
of the sent package if we store the DLC in a private array.

Ideally this should be handled in the can_echo_skb functions, but I
leave that exercise to the CAN folks.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NMarc Kleine-Budde <mkl@pengutronix.de>