can: flexcan: flexcan_rx_offload_setup(): factor out mailbox and rx-offload...

can: flexcan: flexcan_rx_offload_setup(): factor out mailbox and rx-offload setup into separate function

In an upcoming patch the order of operations in flexcan_open() are changed.
Introduce convenience function to make that patch simpler.

Link: https://lore.kernel.org/r/20201119100917.3013281-4-mkl@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>