hwmon: (core) check parent dev != NULL when chip != NULL

hwmon_device_register_with_info() registration API requires a
non-NULL parent device when chip is non-NULL.

This commit adds a check and documents this requirement.

Signed-off-by: Lucas Magasweran <lucas.magasweran@ieee.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>