can: dev: __can_get_echo_skb(): replace struct can_frame by canfd_frame to access frame length

This patch replaces the use of "struct can_frame::can_dlc" by "struct
canfd_frame::len" to access the frame's length. As it is ensured that
both structures have a compatible memory layout for this member this is
no functional change. Futher, this compatibility is documented in a
comment.

Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>