[TTY]: Use tty_mode_ioctl() in network drivers.

We conciously make a change here - we permit mode and speed setting to
be done in things like SLIP mode. There isn't actually a technical
reason to disallow this. It's usually a silly thing to do but we can
do it and soemone might wish to do so.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>