can: tcan4x5x: tcan4x5x_regmap_write(): remove not needed casts and replace 4 by sizeof

This patch simplifies the tcan4x5x_regmap_write(0 function by removing not
needed casts and replaces hardcoded "4" by appropriate sizeof()s.

Reviewed-by: Dan Murphy <dmurphy@ti.com>
Tested-by: Sean Nyekjaer <sean@geanix.com>
Link: https://lore.kernel.org/r/20201215231746.1132907-10-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>