nfp: flower: netdev offload check for ip6gretap

nfp: flower: netdev offload check for ip6gretap

IPv6 GRE tunnels are not being offloaded, this is caused by a missing
netdev offload check. The functionality of IPv6 GRE tunnel offloading
was previously added but this check was not included. Adding the
ip6gretap check allows IPv6 GRE tunnels to be offloaded correctly.

Fixes: a939b21856ca ("nfp: flower: Allow ipv6gretap interface for offloading")
Signed-off-by: Danie du Toit <danie.dutoit@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20220217124820.40436-1-louis.peens@corigine.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>