nfp: wrap napi add/del logic

There will be more NAPI register logic once AF_XDP support is
added, wrap our already conditional napi add/del into helpers.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>