fddi: use ndo_siocdevprivate

The skfddi driver has a private ioctl and passes the data correctly
through ifr_data, but the use of a pointer in s_skfp_ioctl is
broken in compat mode.

Change the driver to use ndo_siocdevprivate and disallow calling
it in compat mode until a conversion handler is added.

Cc: "Maciej W. Rozycki" <macro@orcam.me.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>