acpi_power_meter: drop meter_rw_attrs, use common meter_attrs

We always register these two together, so move meter_rw_attrs into
meter_ro_attrs and use the same for both since we no longer have two
register_attr paths.

Signed-off-by: Kyle McMartin <kyle@redhat.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>