can: m_can: move runtime PM enable/disable to m_can_platform

This is a preparatory patch for upcoming PCI based M_CAN devices. The current
PM implementation would cause PCI based drivers to enable PM twice, once when
the PCI device is added and a second time in m_can_class_register(). This will
cause 'Unbalanced pm_runtime_enable!' to be logged, and is a situation that
should be avoided.

Therefore, in anticipation of PCI devices, move PM enabling out from M_CAN
class registration to its only user, the m_can_platform driver.

Signed-off-by: Patrik Flykt <patrik.flykt@linux.intel.com>
Link: https://lore.kernel.org/r/20201023115800.46538-2-patrik.flykt@linux.intel.com
[mkl: m_can_plat_probe(): fix error handling
      m_can_class_register(): simplify error handling]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>