can: etas_es58x: use sizeof and sizeof_field macros instead of constant values

Replace two constant values by a call to sizeof{,_field} on the
relevant field to make the logic easier to understand.

Link: https://lore.kernel.org/r/20210628155420.1176217-6-mailhol.vincent@wanadoo.fr


Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>