The current implementation uses "wacom_battery" as a generic name for
batteries. This prevents us to have two Wacom devices with a battery
attached as the power system will complain about the name which is already
registered.

Use an incremental name for each battery attached.

Related bug:
https://sourceforge.net/p/linuxwacom/bugs/248/Signed-off-by: NBenjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: NPrzemo Firszt <przemo@firszt.eu>
Acked-by: NPing Cheng <pingc@wacom.com>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>