hamachi: use ndo_siocdevprivate

hamachi has one command that overloads the ifreq argument
and requires a conversion to ndo_siocdevprivate in order to
make compat mode work, so split it from ndo_ioctl.

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