TTY: moxa, convert to dynamic device

This allows us to provide the tty layer with information about
tty_port for each link.

We also provide a tty_port for the service port. For this one we allow
only ioctl, so this is pretty ugly.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>