n_gsm: Send CLD command on exit

A DISC on DLCI 0 should close down the mux but Michael Lauer reports this
is not the case for some modems. Send a CLD as well.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Tested-by: Michael Lauer
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>