iSeries: Fix up viotty_ioctl BKL locking fallout

The bogus code to call into the n_tty layer got removed in commit
8bc5fb6a

 ("Remove bogons from the
iSeries console"), but it left a now uninitialized "return ret;" around.

Not that this code has ever even compiled since the BKL pushdown, since
not only is "ret" no longer initialized, it was never actually declared
even originally.

Replace it with a "return -ENOIOCTLCMD"

Pointed-out-by: Paul Mackerras <paulus@samba.org>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>