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
diff --git a/drivers/power/pm8921-charger.c b/drivers/power/pm8921-charger.c
index 538692a..4acee97 100644
--- a/drivers/power/pm8921-charger.c
+++ b/drivers/power/pm8921-charger.c
@@ -2944,10 +2944,11 @@
 	usb_vin = vchg.physical;
 	pr_info("usb_vin : %d, max_voltage_mv=%d\n", usb_vin, chip->max_voltage_mv);
 
-	if (usb_vin/1000 <= chip->max_voltage_mv) {
-		unplug_ovp_fet_open(chip);
+	if ((usb_vin/1000 <= chip->max_voltage_mv) &&
+		(usb_vin/1000 > PM8921_CHG_VDDMAX_MIN)){
+			pr_info(" Turn off USB ovp \n");
+			unplug_ovp_fet_open(chip);
 	}
-	pr_debug(" Notify USB update here \n");
 	power_supply_changed(&chip->batt_psy);
 	power_supply_changed(&chip->usb_psy);
 	power_supply_changed(&chip->dc_psy);