nfp: create control vNICs and wire up rx/tx

When driver encounters an nfp_app which has a control message handler
defined, allocate a control vNIC.  This control channel will be used
to exchange data with the application FW such as flow table programming,
statistics and global datapath control.

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