can: mcp251xfd: mcp251xfd_probe(): bail out if no IRQ was given

This patch add a check to the mcp251xfd_probe() function to bail out and give
the user a proper error message if no IRQ is specified. Otherwise the driver
will probe just fine but ifup will fail with a meaningless "RTNETLINK answers:
Invalid argument" error message.

Link: https://lore.kernel.org/r/20201123113522.3820052-1-mkl@pengutronix.de
Reported-by: Niels Petter <petter@ka-long.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>