power: supply: lego_ev3_battery: fix Vce offset

This fixes the value that accounts for the Vce of a transistor in the
LEGO MINDSTORMS EV3 power supply driver. The old value (200mV) was the
max value from the data sheet. After testing, the actual value has been
found to be 50mV. By using 50mV we get a more accurate voltage
indication.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>