hwmon: (ibmpowernv) change create_hwmon_attr_name() prototype

It simplifies the creation of the hwmon attributes and will help when
support for a new device tree layout is added. The patch also changes
the name of the routine to parse_opal_node_name().

Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>