ACPI / osl: use same type for acpi_predefined_names values as in definition

In the definition of struct acpi_predefined_names, value is of
type char *. Make the OSL override function also work with type
char * (or, more precisely, with a pointer to it).

Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>