can: esd_usb: Improve code readability by means of replacing struct esd_usb_msg with a union

As suggested by Vincent Mailhol, declare struct esd_usb_msg as a union
instead of a struct. Then replace all msg->msg.something constructs,
that make use of esd_usb_msg, with simpler and prettier looking
msg->something variants.

Link: https://lore.kernel.org/all/CAMZ6RqKRzJwmMShVT9QKwiQ5LJaQupYqkPkKjhRBsP=12QYpfA@mail.gmail.com/


Suggested-by: Vincent MAILHOL <mailhol.vincent@wanadoo.fr>
Signed-off-by: Frank Jungclaus <frank.jungclaus@esd.eu>
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://lore.kernel.org/all/20230222163754.3711766-1-frank.jungclaus@esd.eu


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