greybus: gpio: fix null-deref on unexpected irq requests

Fix null-pointer dereference on failure to look up irq due to missing
error handling.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>