can: dummy_can: dummy_can_init(): fix packet statistics

can: dummy_can: dummy_can_init(): fix packet statistics

The former implementation was only counting the tx_packets value but not
the tx_bytes as the skb was dropped on driver layer.

Enable CAN echo support (IFF_ECHO) in dummy_can_init(), which activates the
code for setting and retrieving the echo SKB and counts the tx_bytes
correctly.

Fixes: 816cf430e84b ("can: add dummy_can driver")
Cc: Vincent Mailhol <mailhol@kernel.org>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Reviewed-by: Vincent Mailhol <mailhol@kernel.org>
Link: https://patch.msgid.link/20260126104540.21024-1-socketcan@hartkopp.net
[mkl: make commit message imperative]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>