can: m_can_platform: m_can_plat_probe(): add missing error handling if mcan_class is NULL

This patch adds the missing error handling in m_can_plat_probe() if
mcan_class is NULL.

Fixes: f524f829

 ("can: m_can: Create a m_can platform framework")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>