usb: chipidea: core: change vbus-regulator as optional

Vbus regualtor is an optional regulator, for platforms, which
doesn't have this regulator, it will get a dummy regulator and
show warning message.
Signed-off-by: Peter Chen <peter.chen@nxp.com>

drivers/usb/chipidea/core.c

@@ -683,7 +683,7 @@ static int ci_get_platdata(struct device *dev,
 
 	if (platdata->dr_mode != USB_DR_MODE_PERIPHERAL) {
 		/* Get the vbus regulator */
-		platdata->reg_vbus = devm_regulator_get(dev, "vbus");
+		platdata->reg_vbus = devm_regulator_get_optional(dev, "vbus");
 		if (PTR_ERR(platdata->reg_vbus) == -EPROBE_DEFER) {
 			return -EPROBE_DEFER;
 		} else if (PTR_ERR(platdata->reg_vbus) == -ENODEV) {