HID: wacom: Status packet provides 'charging', not 'powered' bit

The status packet for tablets which can use a wireless module contains a
bit that is set if the battery is charging. This bit will be 0 if either
a battery is not present or if the battery has reached full charge. Note
that the charging circuit may continue to charge the battery for a short
time after reaching "100%".

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>