ACPI: Don't report current_now if battery reports in mWh

ACPI batteries can report in units of either current or energy. Right
now we expose the current_now file even if the battery is reporting
energy units, resulting in a file that should contain mA instead
containing mW. Don't expose this value unless the battery is reporting
current.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>