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

drivers/net/can/m_can/tcan4x5x-regmap.c

@@ -76,7 +76,7 @@ static const struct regmap_config tcan4x5x_regmap = {
 	.max_register = TCAN4X5X_MAX_REGISTER,
 };
 
-static struct regmap_bus tcan4x5x_bus = {
+static const struct regmap_bus tcan4x5x_bus = {
 	.write = tcan4x5x_regmap_write,
 	.gather_write = regmap_spi_gather_write,
 	.async_write = regmap_spi_async_write,