s390/tty3270: move ASCII->EBCDIC conversion to convert_line()

Instead of always converting the character set, only convert them
when the line is really displayed.

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>