can: c_can: use 32-bit write to set arbitration register

The arbitration register is already set up with 32-bit writes in the
other parts of the code except for this point.

Link: https://lore.kernel.org/r/20210302215435.18286-5-dariobin@libero.it
Signed-off-by: Dario Binacchi <dariobin@libero.it>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>