nfp: fix unchecked flow dissector use

Previously flow dissectors were referenced without first checking that
they are in use and correctly populated by TC. This patch fixes this by
checking each flow dissector key before referencing them.

Fixes: 5571e8c9 ("nfp: extend flower matching capabilities")
Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>