Open USB log in factory version for bangkk[3/3]

In order to track the usb abnormal disconnect problem,
open some log about usb in factory version recommended
by Qualcomm for bangkk.

Change-Id: If317121e650bd48fd09e50825ecd3fa83cc77c02
Reviewed-on: https://gerrit.mot.com/2654715
SME-Granted: SME Approvals Granted
SLTApproved: Slta Waiver
Tested-by: Jira Key
Reviewed-by: Zonghua Liu <a17671@motorola.com>
Reviewed-by: Xiangpo Zhao <zhaoxp3@motorola.com>
Submit-Approved: Jira Key

drivers/moto_f_usbnet/f_usbnet.c

@@ -1001,6 +1001,9 @@ static void usbnet_disable(struct usb_function *f)
 	struct usbnet_device  *dev = usbnet_func_to_dev(f);
 	struct usbnet_context *context = dev->net_ctxt;
 	USBNETDBG(context, "%s\n", __func__);
+#ifdef CONFIG_MOT_USB_DEBUG_LOG
+	dump_stack();
+#endif
 	do_set_config(f, 0);
 }