usb: gadget: rndis: Fix re-binding f_rndis

f_rndis checks if rndis_string_defs[0].id is null to setup rndis
and allocate string ids when it is bound to the first configuration:

	/* maybe allocate device-global string IDs */
	if (rndis_string_defs[0].id == 0) {

		/* ... and setup RNDIS itself */
		status = rndis_init();
		if (status < 0)
			return status;

rndis_string_defs[0].id must be reset to 0 on unbind for rndis to be
correctly initialized on the next composite_bind.

Signed-off-by: Benoit Goby <benoit@android.com>
diff --git a/drivers/usb/gadget/f_rndis.c b/drivers/usb/gadget/f_rndis.c
index 7b1cf18..f377c10 100644
--- a/drivers/usb/gadget/f_rndis.c
+++ b/drivers/usb/gadget/f_rndis.c
@@ -819,6 +819,7 @@
 
 	rndis_deregister(rndis->config);
 	rndis_exit();
+	rndis_string_defs[0].id = 0;
 
 	if (gadget_is_superspeed(c->cdev->gadget))
 		usb_free_descriptors(f->ss_descriptors);