s390/con3270: optionally disable auto update

This patch adds a parameter 'auto_update' to the con3270
driver, causing the 'auto_update' feature to be disabled
if unset.
The 'auto_update' feature will cause the con3270 driver
to switch to the console view whenever new system messages
are displayed, which makes working on the 3270 terminal
awkward.

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