s390/con3270: move tty3270_convert_line()

To make the upcoming patches easier to read, move tty3270_convert_line()
before changing code. No functional change.

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>