pmu_battery: Fix battery full reporting

Prior to this patch, pmu_battery was unable to report battery full
status. This patch fixes the issue by adding a proper handling code
into pmu_bat_get_property(): if we're on AC and the battery isn't
charging, then the battery is considered full.

Signed-off-by: Thomas Champagne <lafeuil@gmail.com>
Acked-By: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>