power: supply: olpc_battery: Move priv data to a struct

The global variables for private data are not too nice. I'd like some
more, and that would clutter the global name space even further.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>