HID: wacom: Centralize updating of wacom_wac battery status

Has the 'wacom_notify_battery' function take on the job of detecting if
updating the power supply is necessary to remove multiple
nearly-identical 'if' blocks.

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