nfp: move physical port init into a helper

Move MAC/PHY port init into a helper to make it easier to reuse
it in the representor code.

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