nfp: prepare config and enable for working without netdevs

Out of the three stages of ifup/ifdown (allocate, configure, start)
- this commit prepares the configuration stage for working with
control vNICs.

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