can: esd_usb: Replace hardcoded message length given to USB commands

Replace all hardcoded values supplied to the len element of
esd_usb_msg (and its siblings) by more readable expressions, based on
sizeof(), offsetof(), etc.
Also spend documentation / comments that the len element of esd_usb_msg
is in multiples of 32bit words and not in bytes.

Link: https://lore.kernel.org/all/CAMZ6RqLaDNy-fZ2G0+QMhUEckkXLL+ZyELVSDFmqpd++aBzZQg@mail.gmail.com/


Suggested-by: Vincent MAILHOL <mailhol.vincent@wanadoo.fr>
Signed-off-by: Frank Jungclaus <frank.jungclaus@esd.eu>
Link: https://lore.kernel.org/r/20230519195600.420644-6-frank.jungclaus@esd.eu


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