HID: wacom: remove the battery when the EKR is off

Currently the EKR battery remains even after we stop getting information
from the device. This can lead to a stale battery persisting indefinitely
in userspace.

The remote sends a heartbeat every 10 seconds. Delete the battery if we
miss two heartbeats (after 21 seconds). Restore the battery once we see
a heartbeat again.

Signed-off-by: Aaron Skomra <skomra@gmail.com>
Signed-off-by: Aaron Armstrong Skomra <aaron.skomra@wacom.com>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Fixes: 9f1015d4 ("HID: wacom: EKR: attach the power_supply on first connection")
CC: stable@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>