power: charger: set under threshold as vddmax_min for unplug_usbcheck_work.

When usb mode is changed from MTP to Tethering, chg_gone_irq_handler is called
and then disconnect tethering by turning off ovpfet from unplug_ovp_fet_open.
So we add under threshold to protect this error while usb is being connected.

Change-Id: Id66a3db0822c291ba348cf93d2e7a72f6ae857ae