Both struct tcan4x5x_priv::mram_start and struct tcan4x5x_priv::reg_offset are
only assigned once with a constant and then always used read-only. This patch
changes the driver to use the constant directly instead.

Link: https://lore.kernel.org/r/20201130133713.269256-2-mkl@pengutronix.deReviewed-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>