nfp: flower: handle neighbour events on internal ports

Recent code changes to NFP allowed the offload of neighbour entries to FW
when the next hop device was an internal port. This allows for offload of
tunnel encap when the end-point IP address is applied to such a port.

Unfortunately, the neighbour event handler still rejects events that are
not associated with a repr dev and so the firmware neighbour table may get
out of sync for internal ports.

Fix this by allowing internal port neighbour events to be correctly
processed.

Fixes: 45756dfe

 ("nfp: flower: allow tunnels to output to internal port")
Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>