s390/3270: handle reconnect of a tty with a different size

If an existing 3270 terminal disconnects and reconnects with a
different size, the 3270 driver still works with the old size.
If the new dimensions are larger the output merely looks funny,
if the new dimensions are smaller the terminal is unusable.

To fix this restart the size detection after a unit check has
been received.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>