eql: use ndo_siocdevprivate

The private ioctls in eql pass the arguments correctly through ifr_data,
but the slaving_request_t and slave_config_t structures are incompatible
with compat mode and need special conversion code in the driver.

Convert to siocdevprivate for now, and return an error when called
in compat mode.

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