tty: n_gsm: update doc example to use header for N_GSM0710 define

There is no reason to gues the line discipline number when it is
available from tty.h

Signed-off-by: Martin Hundebøll <martin@geanix.com>
Link: https://lore.kernel.org/r/20190710192656.60381-2-martin@geanix.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>