nfp: add support for tx/rx with metadata portid

Allow tx/rx with metadata port id. This will be used for tx/rx of
representor netdevs acting as upper-devices while a pf netdev acts
as a lower-device.

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