can: m_can: move Message RAM initialization to function

To avoid possible ECC/parity checksum errors when reading an
uninitialized buffer, the entire Message RAM is initialized when probing
the driver. This initialization is done in the same function reading the
Device Tree properties.

This patch moves the RAM initialization to a separate function so it can
be called separately from device initialization from Device Tree.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>