power: supply: axp288_charger: Fix initial constant_charge_current value

We should look at val which contains the value read from the register,
not ret which is always 0 on a successful read.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Fixes: eac53b36 ("power: supply: axp288_charger: Drop platform_data dependency")
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>