can: rcar_canfd: Share config code in rcar_canfd_set_bittiming()

The configuration register format for nominal bit timings in CAN-FD mode
and the format for bit timings in CAN mode on CAN-FD controllers with
shared Classical CAN registers are the same.

Restructure the code to make this clear, also reducing kernel size by 80
bytes.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://patch.msgid.link/b7643a3c49777989d02145a85b85cf773ec2123f.1749655315.git.geert+renesas@glider.be


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