The Janz VMOD-ICAN3 firmware does not support any sort of TX-done
notification or interrupt. The driver previously used the hardware
loopback to attempt to work around this deficiency, but this caused all
sockets to receive all messages, even if CAN_RAW_RECV_OWN_MSGS is off.

Using the new function ican3_cmp_echo_skb(), we can drop the loopback
messages and return the original skbs. This fixes the issues with
CAN_RAW_RECV_OWN_MSGS.

A private skb queue is used to store the echo skbs. This avoids the need
for any index management.

Due to a lack of TX-error interrupts, bus errors are permanently
enabled, and are used as a TX-error notification. This is used to drop
an echo skb when transmission fails. Bus error packets are not generated
if the user has not enabled bus error reporting.
Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>