can: m_can: m_can_class_free_dev(): introduce new function

This patch creates a common function that peripherials can call to free the
netdev device when failures occur.

Fixes: f524f829

 ("can: m_can: Create a m_can platform framework")
Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Link: http://lore.kernel.org/r/20200227183829.21854-2-dmurphy@ti.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>