s390/vmcp: return -ENOTTY for unknown ioctl commands

Return -ENOTTY for unknown ioctl commands instead of
-ENOIOCTLCMD. This isn't that much of difference, since common code
will translate -ENOIOCTLCMD to -ENOTTY anyway, but this way it seems
to be more obvious what is happening (at least to me).

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>