s390/tty3270: add 3270 datastream helpers

There are lots of places adding attributes or orders to the datastream.
Add a few helpers to make that code shorter and easier to read.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>