can: c_can: make {read,write}_reg functions const

This patch makes the {read,write}_reg functions const, this is a preparation to
make use of {read,write}_reg in the hwinit callback.

Signed-off-by: Thor Thayer <tthayer@altera.com>
Signed-off-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>