usb: fix dwc3 build when USB_GADGET_DWC3 is not enabled

Fix build error when CONFIG_USB_GADGET_DWC3 is not enabled:

ERROR: "dwc3_send_gadget_ep_cmd" [drivers/usb/dwc3/dwc3.ko] undefined!
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/dwc3/gadget.h

@@ -202,6 +202,11 @@ void dwc3_gadget_exit(struct dwc3 *dwc);
 #else
 static inline int dwc3_gadget_init(struct dwc3 *dwc) { return 0; }
 static inline void dwc3_gadget_exit(struct dwc3 *dwc) { }
+static inline int dwc3_send_gadget_ep_cmd(struct dwc3 *dwc, unsigned ep,
+		unsigned cmd, struct dwc3_gadget_ep_cmd_params *params)
+{
+	return 0;
+}
 #endif
 
 void dwc3_gadget_giveback(struct dwc3_ep *dep, struct dwc3_request *req,