can: m_can: fix periph RX path: use rx-offload to ensure skbs are sent from softirq context

For peripheral devices, m_can sent skbs directly from a threaded irq
instead of from a softirq context, breaking the tcan4x5x peripheral
driver completely. This patch transitions the driver to use the
rx-offload helper for peripherals, ensuring the skbs are sent from the
correct context, with h/w timestamping to ensure correct ordering.

Link: https://lore.kernel.org/r/20210308102427.63916-4-torin@maxiluxsystems.com
Signed-off-by: Torin Cooper-Bennun <torin@maxiluxsystems.com>
[mkl: m_can_class_register(): update error handling]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>