HID: wiimote: use cached battery values on I/O failure

Battery reports are sent along every status report of the Wii Remote.
So chances are pretty high that we have an up-to-date battery
cache at any time. Therefore, initialize the battery-cache to 100% and
then return battery values from the cache if the query fails.

This works around a power_supply limitation in that it requires us to be
able to query the device during power_supply registration and
removal. Otherwise, "add" or "remove" udev events are not sent. If
we answer these requests from our cache instead, we avoid dropping these
events and no longer cause warnings printed.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>