can: m_can: m_can_class_allocate_dev(): remove impossible error return judgment

If the CAN net device has been successfully allocated, its private
data structure is impossible to be empty, remove this redundant error
return judgment.

Link: https://lore.kernel.org/r/20210205072559.13241-2-xulin.sun@windriver.com
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>