power: supply: olpc_battery: Hold the reference returned by of_find_compatible_node

In olpc_battery_probe(), we should hold the reference returned by
of_find_compatible_node() and use it to call of_node_put() for
refcount balance.

Signed-off-by: Liang He <windhl@126.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>