usb: dwc3: dwc3-omap: return -EPROBE_DEFER if probe has not yet executed

return -EPROBE_DEFER from dwc3_omap_mailbox in dwc3-omap.c, if the probe of
dwc3-omap has not yet been executed or failed.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/include/linux/usb/dwc3-omap.h b/include/linux/usb/dwc3-omap.h
index 51eae14..5615f4d 100644
--- a/include/linux/usb/dwc3-omap.h
+++ b/include/linux/usb/dwc3-omap.h
@@ -19,11 +19,11 @@
 };
 
 #if (defined(CONFIG_USB_DWC3) || defined(CONFIG_USB_DWC3_MODULE))
-extern void dwc3_omap_mailbox(enum omap_dwc3_vbus_id_status status);
+extern int dwc3_omap_mailbox(enum omap_dwc3_vbus_id_status status);
 #else
-static inline void dwc3_omap_mailbox(enum omap_dwc3_vbus_id_status status)
+static inline int dwc3_omap_mailbox(enum omap_dwc3_vbus_id_status status)
 {
-	return;
+	return -ENODEV;
 }
 #endif