s390x/sclpconsole-lm: truncate input if line is too long

As the SCLP line mode console input length is limited by the available
SCCB buffer space, it might lock up if the input does not fit into the
buffer.

With this patch, characters that don't fit are 'eaten' up to the next
CR/LF and the input line is sent truncated to the guest.

Signed-off-by: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
