can: isotp: isotp_setsockopt(): only allow to set low level TX flags for CAN-FD

can: isotp: isotp_setsockopt(): only allow to set low level TX flags for CAN-FD

CAN-FD frames have struct canfd_frame::flags, while classic CAN frames
don't.

This patch refuses to set TX flags (struct
can_isotp_ll_options::tx_flags) on non CAN-FD isotp sockets.

Fixes: 3f9425168a78 ("can: add ISO 15765-2:2016 transport protocol")
Link: https://lore.kernel.org/r/20210218215434.1708249-2-mkl@pengutronix.de
Cc: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>