virtio/s390: keep early_put_chars

In case the registration of the hvc tty never happens AND the kernel
thinks that hvc0 is the preferred console we should keep the early
printk function to avoid a kernel panic due to code being removed.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Jing Liu <liujbjl@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>