hwmon: (sch5627) Trigger Vbat measurements

The sch5627 needs to be explicitly told to start an adc conversion
for Vbat, once in a while. Without this Vbat may read 0, and will never
get updated.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>