can: etas_es58x: use BITS_PER_TYPE() instead of manual calculation

The input to the GENMASK() macro was calculated by hand. Replaced it
with a dedicated macro: BITS_PER_TYPE() which does the exact same job.

Link: https://lore.kernel.org/all/20220212130737.3008-1-mailhol.vincent@wanadoo.fr


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