s390/con3270: add helper to get number of tty rows

There a quite a few places using 'tp->view.rows - 2'.
Add a helper function for this. This will also be used
when a function key help line will be added.

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>