can: tcan4x5x: tcan4x5x_can_probe(): add missing error handling if mcan_class is NULL

This patch adds the missing error handling in tcan4x5x_can_probe() if
mcan_class is NULL.

Fixes: 5443c226

 ("can: tcan4x5x: Add tcan4x5x driver to the kernel")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>