s390/3270: fix garbled output on 3270 tty view

The tty3270_update function tries to optimize the output stream by
skipping the TO_SBA command to position the output offset if the
TO_RA command of the previous line ended at the same offset.
But some lines do not have the TO_RA at the end, namely those
with a length equal to the number of columns.

Make sure there is a TO_RA at the end of the previous line to avoid
funny looking output.

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