wan: use ndo_siocdevprivate

The wan drivers each support some custom SIOCDEVPRIVATE
ioctls, plus the common SIOCWANDEV command.

Split these so the ioctl callback only deals with SIOCWANDEV
and the rest is handled by ndo_siocdevprivate.

It might make sense to also split out SIOCWANDEV into a
separate callback in order to eventually remove ndo_do_ioctl
entirely.

Cc: Krzysztof Halasa <khc@pm.waw.pl>
Cc: Kevin Curtis <kevin.curtis@farsite.co.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>