usb: acm gadget: Null termintate strings table

The gadget strings table should be null terminated.
usb_gadget_get_string() loops through the table
expecting a null at the end of the list.
Signed-off-by: Graham Williams <gwilli@broadcom.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/f_acm.c

@@ -285,6 +285,7 @@ static struct usb_string acm_string_defs[] = {
 	[ACM_CTRL_IDX].s = "CDC Abstract Control Model (ACM)",
 	[ACM_DATA_IDX].s = "CDC ACM Data",
 	[ACM_IAD_IDX ].s = "CDC Serial",
+	{  } /* end of list */
 };
 
 static struct usb_gadget_strings acm_string_table = {