power: supply: max17042_battery: Refactor max17042_external_power_changed()

max17042_external_power_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.

Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Sebastian Krzyszkowiak <sebastian.krzyszkowiak@puri.sm>
Cc: Purism Kernel Team <kernel@puri.sm>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>