tty: n_gsm: remove obsolete mknod doc example

The n_gsm driver handles registration of /dev/gsmttyX nodes, so there's
no need to do mknod manually.

Signed-off-by: Martin Hundebøll <martin@geanix.com>
Link: https://lore.kernel.org/r/20190710192656.60381-1-martin@geanix.com


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