can: rcar_canfd: Simplify clock handling

The main CAN clock is either the internal CANFD clock, or the external
CAN clock.  Hence replace the two-valued enum by a simple boolean flag.
Consolidate all CANFD clock handling inside a single branch.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://lore.kernel.org/all/2cf38c10b83c8e5c04d68b17a930b6d9dbf66f40.1716973640.git.geert+renesas@glider.be


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