nfp: flower: refactor tunnel key layer calculation

Refactor the key layer calculation function, in particular the tunnel
key layer calculation by introducing helper functions. This is done
in preparation for supporting GRE tunnel offloads.

Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>