usb: dwc2: dwc2_vbus_supply_init: fix error check

devm_regulator_get_optional returns -ENODEV if the regulator isn't
there, so if that's the case we have to make sure not to leave -ENODEV
in the regulator pointer.

Also, make sure we return 0 in that case, but correctly propagate any
other errors. Also propagate the error from _dwc2_hcd_start.

Fixes: 531ef5ebea ("usb: dwc2: add support for host mode external vbus supply")
Cc: Amelie Delaunay <amelie.delaunay@st.com>
Reviewed-by: Amelie Delaunay <amelie.delaunay@st.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc2/hcd.c

@@ -358,9 +358,14 @@ static void dwc2_gusbcfg_init(struct dwc2_hsotg *hsotg)
 
 static int dwc2_vbus_supply_init(struct dwc2_hsotg *hsotg)
 {
+	int ret;
+
 	hsotg->vbus_supply = devm_regulator_get_optional(hsotg->dev, "vbus");
-	if (IS_ERR(hsotg->vbus_supply))
-		return 0;
+	if (IS_ERR(hsotg->vbus_supply)) {
+		ret = PTR_ERR(hsotg->vbus_supply);
+		hsotg->vbus_supply = NULL;
+		return ret == -ENODEV ? 0 : ret;
+	}
 
 	return regulator_enable(hsotg->vbus_supply);
 }
@@ -4342,9 +4347,7 @@ static int _dwc2_hcd_start(struct usb_hcd *hcd)
 
 	spin_unlock_irqrestore(&hsotg->lock, flags);
 
-	dwc2_vbus_supply_init(hsotg);
-
-	return 0;
+	return dwc2_vbus_supply_init(hsotg);
 }
 
 /*