hwmon: (ibmpowernv) add a convert_opal_attr_name() routine

It simplifies the create_hwmon_attr_name() routine and it clearly isolates
the conversion done between the OPAL node names and hwmon attributes names.

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