can: flexcan: flexcan_set_bittiming(): move setup of CAN-2.0 bitiming into separate function

This is a patch prepares for the CAN-FD support. In a later patch the
setup for canfd bittiming will be added, with this patch the change is
easier to read.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20200922144429.2613631-14-mkl@pengutronix.de