serport: place serport serio device correctly in the device tree

Make serport serio device to be a child of corresponding tty device
instead of just hanging at /sys/devices/serioX.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Acked-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>