can: rcar_can: fix IRQ check

rcar_can_probe() regards 0 as a wrong IRQ #, despite platform_get_irq() that it
calls returns negative error code in that case. This leads to the following
being printed to the console when attempting to open the device:

error requesting interrupt fffffffa

because  rcar_can_open() calls request_irq() with a negative IRQ #, and that
function naturally fails with -EINVAL.

Check for the negative error codes instead and propagate them upstream instead
of just returning -ENODEV.

Fixes: fd115931

 ("can: add Renesas R-Car CAN driver")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>