platform/chrome: cros_ec: Do not attempt to register a non-positive IRQ number

Add a layer of sanity checking to cros_ec_register against attempting to
register IRQ values that are not strictly greater than 0.

Signed-off-by: Enrico Granata <egranata@chromium.org>
Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>