nfp: add control vNIC datapath

Since control vNICs don't have a netdev, they can't use napi and
queuing stack provides.  Add simple tasklet-based data receive
and send of control messages with queuing on a skb_list.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>