nfp: don't advertise hw-tc-offload on non-port netdevs

nfp_port is a structure which represents an ASIC port, both
PCIe vNIC (on a PF or a VF) or the external MAC port.  vNIC
netdev (struct nfp_net) and pure representor netdev (struct
nfp_repr) both have a pointer to this structure.  nfp_reprs
always have a port associated.  nfp_nets, however, only represent
a device port in legacy mode, where they are considered the
MAC port. In switchdev mode they are just the CPU's side of
the PCIe link.

By definition TC offloads only apply to device ports.  Don't
set the flag on vNICs without a port (i.e. in switchdev mode).

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Tested-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>