s390/con3270: move condev definition

Fix this for allmodconfig:

drivers/s390/char/con3270.c:43:24: error: 'condev' defined but not used [-Werror=unused-variable]
 static struct tty3270 *condev;
                        ^~~~~~

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: c17fe081 ("s390/3270: unify con3270 + tty3270")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>