can: m_can: use cdev as name for struct m_can_classdev uniformly

This patch coverts the m_can driver to use cdev as name for struct
m_can_classdev uniformly throughout the whole driver.

Link: https://lore.kernel.org/r/20201212175518.139651-4-mkl@pengutronix.de


Reviewed-by: Sean Nyekjaer <sean@geanix.com>
Reviewed-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>