USB: gadget: Mark ep driver_data as NULL after ep disable

After disabling the endpoint need to mark driver_data of that
endpoint as NULL, else cannot claim the same endpoint during
composition switch.

CRs-fixed: 381861
Change-Id: I8b008d288c110d0feee820557aab3fcb869b5972
Signed-off-by: Chiranjeevi Velempati <cvelempa@codeaurora.org>
diff --git a/drivers/usb/gadget/f_serial.c b/drivers/usb/gadget/f_serial.c
index 7b6acc6..3d6ceaa 100644
--- a/drivers/usb/gadget/f_serial.c
+++ b/drivers/usb/gadget/f_serial.c
@@ -572,6 +572,7 @@
 #ifdef CONFIG_MODEM_SUPPORT
 	usb_ep_fifo_flush(gser->notify);
 	usb_ep_disable(gser->notify);
+	gser->notify->driver_data = NULL;
 #endif
 	gser->online = 0;
 }