can: tcan4x5x: mark struct regmap_bus tcan4x5x_bus as constant

This patch marks the struct regmap_bus tcan4x5x_bus as constant.

Reviewed-by: Dan Murphy <dmurphy@ti.com>
Tested-by: Sean Nyekjaer <sean@geanix.com>
Link: https://lore.kernel.org/r/20201215231746.1132907-6-mkl@pengutronix.de


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