usb: dwc2: get optional vbus-supply regulator once

Move devm_regulator_get_optional() call to probe routine. This avoids
'vbus-supply' regulator to be requested lots of times, upon each call
to dwc2_vbus_supply_init(), e.g. like with runtime pm.

Fixes: 531ef5ebea96 ("usb: dwc2: add support for host mode external
vbus supply")

Tested-by: Artur Petrosyan <arturp@synopsys.com>
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
diff --git a/drivers/usb/dwc2/hcd.c b/drivers/usb/dwc2/hcd.c
index 5f23b93..24aa5a3 100644
--- a/drivers/usb/dwc2/hcd.c
+++ b/drivers/usb/dwc2/hcd.c
@@ -358,16 +358,10 @@ static void dwc2_gusbcfg_init(struct dwc2_hsotg *hsotg)
 
 static int dwc2_vbus_supply_init(struct dwc2_hsotg *hsotg)
 {
-	int ret;
+	if (hsotg->vbus_supply)
+		return regulator_enable(hsotg->vbus_supply);
 
-	hsotg->vbus_supply = devm_regulator_get_optional(hsotg->dev, "vbus");
-	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);
+	return 0;
 }
 
 static int dwc2_vbus_supply_exit(struct dwc2_hsotg *hsotg)