Input: wacom - battery reporting improvements

Do not register battery device until connected to a tablet.
This prevents an empty battery icon from being shown when tablet is
connected using USB cable.

Also, call power_supply_powers() for apps that can make use of that
info.

And stop ignoring input registration failures.

Signed-off-by: Chris Bagwell <chris@cnpbagwell.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>