tehuti: use ndo_siocdevprivate

Tehuti only implements private ioctl commands, and implements
them by overriding the ifreq layout, which is broken in
compat mode.

Move it to the ndo_siocdevprivate callback in order to fix this.

Cc: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>