can: rx-offload: can_rx_offload_add_manual(): add new initialization function

can: rx-offload: can_rx_offload_add_manual(): add new initialization function

This patch adds a new initialization function:
can_rx_offload_add_manual()

It should be used to add support rx-offload to a driver, if the callback
mechanism should not be used. Use e.g. can_rx_offload_queue_sorted() to queue
skbs into rx-offload.

Link: https://lore.kernel.org/r/20200915223527.1417033-33-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>