hwmon: (pmbus_core) Fix compiler warning

Some configurations produce the following compiler warning:

drivers/hwmon/pmbus/pmbus_core.c: In function 'pmbus_show_boolean':
drivers/hwmon/pmbus/pmbus_core.c:752: warning: 'val' may be used uninitialized in this function

While this is a false positive, it can easily be fixed by overloading the return
value from pmbus_get_boolean with both val and error return code (val is a
boolean and thus never negative).

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>