Round constant_charge_voltage writes down to the first supported lower
value, rather then rounding them up to the first supported higher value.

This fixes e.g. writing 4250000 resulting in a value of 4350000 which
might be dangerous, instead writing 4250000 will now result in a safe
4200000 value.

Fixes: 843735b7 ("power: axp288_charger: axp288 charger driver")
Cc: stable@vger.kernel.org
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20240717200333.56669-2-hdegoede@redhat.comSigned-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>