can: esd_usb: Improve readability on decoding ESD_EV_CAN_ERROR_EXT messages

As suggested by Marc introduce a union plus a struct ev_can_err_ext
for easier decoding of an ESD_EV_CAN_ERROR_EXT event message (which
simply is a rx_msg with some dedicated data).

Suggested-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/linux-can/20220621071152.ggyhrr5sbzvwpkpx@pengutronix.de/


Signed-off-by: Frank Jungclaus <frank.jungclaus@esd.eu>
Link: https://lore.kernel.org/all/20230216190450.3901254-4-frank.jungclaus@esd.eu


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>