n_gsm: Fix length handling

If the mux is configured with a large mru/mtu the existing code gets the
byte ordering wrong for the header.

Signed-off-by: Ken Mills <ken.k.mills@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>