nfp: add ethtool statistics for representors

Representors may be associated with both VFs or more importantly
with physical ports.  Allow vNIC and MAC statistics to be read
with ethtool -S on representors.  In case of vNICs we reuse
the vNIC statistic helper, we just need to swap RX and TX to
give statistics the "switch perspective."

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