power: supply: bq256xx: Init ichg/vbat value with chip default value

Init the ichg/vbat reg with chip default value instead of the max value
used now. The max value set in driver will result an unsafe case (e.g.
battery is over charging when in a hot environment) if no user space
update the value later.

Signed-off-by: Hermes Zhang <chenhuiz@axis.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>