hwmon: (acpi_power_meter) Fix fail to load module on platform without _PMD method

According to the ACPI specification, the _PMD method is optional. The
acpi_power_meter driver shouldn't fail to load if the platform has no
_PMD method.

Signed-off-by: Huisong Li <lihuisong@huawei.com>
Message-ID: <20241112021228.22914-1-lihuisong@huawei.com>
[groeck: Reworded commit description]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>