can: rcar_can: Fix erroneous registration

Assigning 2 to "renesas,can-clock-select" tricks the driver into
registering the CAN interface, even though we don't want that.
This patch improves one of the checks to prevent that from happening.

Fixes: 862e2b6a ("can: rcar_can: support all input clocks")
Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>