nfp: fix supported key layers calculation

Previously when calculating the supported key layers MPLS, IPv4/6
TTL and TOS were not considered. This patch checks that the TTL and
TOS fields are masked out before offloading. Additionally this patch
checks that MPLS packets are correctly handled, by not offloading them.

Fixes: af9d842c ("nfp: extend flower add flow offload")
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>