phonet: use siocdevprivate

phonet has a single private ioctl that is broken in compat
mode on big-endian machines today because the data returned
from it is never copied back to user space.

Move it over to the ndo_siocdevprivate callback, which also
fixes the compat issue.

Cc: Remi Denis-Courmont <courmisch@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Rémi Denis-Courmont <courmisch@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>