nfp: make sure debug accesses don't depend on netdevs

We want to be able to inspect the state of descriptor rings of
the control vNIC, so it will use the same interface as data vNICs.

Make sure the code doesn't use netdevs to determine state
of the rings and names things appropriately.

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