nfp: provide nfp_port to of nfp_net_get_mac_addr()

Provide port rather than vNIC as parameter of nfp_net_get_mac_addr.
This is to allow this function to be used by representor netdevs where
a vNIC may have more than one physical port none of which are associated
with the vNIC.

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>