s390/con3270: fix multiple assignments in one line

fix the following and similar checkpatch warnings:

CHECK: multiple assignments should be avoided
+		       tp->cx = tp->saved_cx = 0;

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