nfp: return -EOPNOTSUPP from .ndo_get_phys_port_name for VFs

After recent change we started returning 0 from
ndo_get_phys_port_name for VFs.  The name parameter for
ndo_get_phys_port_name is not initialized by the stack so
this can lead to a crash.  We should have kept returning
-EOPNOTSUPP in the first place.

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