s390/configs: change CONFIG_VIRTIO_CONSOLE to "m"

In former times, the virtio-console code had to be compiled into
the kernel since the old guest virtio transport had some hard de-
pendencies. But since the old virtio transport has been removed in
commit 7fb2b2d5 ("s390/virtio: remove the old KVM virtio transport"),
we do not have this limitation anymore.
Commit bb533ec8

 ("s390/config: do not select VIRTIO_CONSOLE via
Kconfig") then also lifted the hard setting in the Kconfig system, so
we can finally switch the CONFIG_VIRTIO_CONSOLE knob to compile this
driver as a module now, making it more flexible for the user to only
load it if it is really required.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/r/20210428082442.321327-1-thuth@redhat.com
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>