nfp: bpf: reject TC offload if XDP loaded

Recent TC changes dropped the check protecting us from trying
to offload a TC program if XDP programs are already loaded.

Fixes: 90d97315 ("nfp: bpf: Convert ndo_setup_tc offloads to block callbacks")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>