cxgb3: use ndo_siocdevprivate

cxgb3 has a private multiplexor that works correctly in compat
mode, split out the siocdevprivate callback from do_ioctl for
simplification.

Cc: Raju Rangoju <rajur@chelsio.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>