nfp: reintroduce ndo_get_port_parent_id for representor ports

NFP does not register devlink ports for representors (without
the "devlink: expose PF and VF representors as ports" series
there are no port flavours to expose them as).

Commit c25f08ac ("nfp: remove ndo_get_port_parent_id implementation")
went to far in removing ndo_get_port_parent_id for representors.
This causes redirection offloads to fail, and switch_id attribute
missing.

Reintroduce the ndo_get_port_parent_id callback for representor ports.

Fixes: c25f08ac ("nfp: remove ndo_get_port_parent_id implementation")
Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>