can: gw: give feedback on missing CGW_FLAGS_CAN_IIF_TX_OK flag

To send CAN traffic back to the incoming interface a special flag has to
be set. When creating a routing job for identical interfaces without this
flag the rule is created but has no effect.

This patch adds an error return value in the case that the CAN interfaces
are identical but the CGW_FLAGS_CAN_IIF_TX_OK flag was not set.

Reported-by: Jannik Hartung <jannik.hartung@tu-bs.de>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://lore.kernel.org/all/20230125055407.2053-1-socketcan@hartkopp.net
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>