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>