can: c_can: Simplify buffer reenabling

Instead of writing to the message object we can simply clear the
NewDat bit with the get method.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>