power: supply: sbs-battery: relax voltage limit

The Smart Battery Data Specification allows for values 0..65535 mV,
there is no reason to limit the value to 20000.

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>