nfp: move repr handling on RX path

Representor packets are received on PF queues with special metadata tag
for demux.  There is no reason to resolve the representor ID -> netdev
after the skb has been allocated.  Move the code, this will allow us to
handle special FW messages without SKB allocation overhead.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>