power: supply: twl4030_madc_battery: Refactor twl4030_madc_bat_ext_changed()

twl4030_madc_bat_ext_changed() is a wrapper around
"power_supply_changed(psy);" and it has the same prototype.

Remove it, replacing it with making the external_power_changed
callback directly point to power_supply_changed.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>