Fix the following compile warning with C=1:

| drivers/net/can/rcar/rcar_canfd.c:1852:59: warning: Using plain integer as NULL pointer

Fixes: a0340df7 ("can: rcar_canfd: Add transceiver support")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20230328145658.7fdbc394@kernel.orgSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Link: https://lore.kernel.org/all/7f7b0dde0caa2d2977b4fb5b65b63036e75f5022.1680071972.git.geert+renesas@glider.beSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>