nfp: forbid disabling hw-tc-offload on representors while offload active

All netdevs which can accept TC offloads must implement
.ndo_set_features().  nfp_reprs currently do not do that, which
means hw-tc-offload can be turned on and off even when offloads
are active.

Whether the offloads are active is really a question to nfp_ports,
so remove the per-app tc_busy callback indirection thing, and
simply count the number of offloaded items in nfp_port structure.

Fixes: 8a276873

 ("nfp: provide infrastructure for offloading flower based TC filters")
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>