nfp: flower: Allow ipv6gretap interface for offloading

The tunnel_type check only allows for "netif_is_gretap", but for
OVS the port is actually "netif_is_ip6gretap" when setting up GRE
for ipv6, which means offloading request was rejected before.

Therefore, adding "netif_is_ip6gretap" allow ipv6gretap interface
for offloading.

Signed-off-by: Yu Xiao <yu.xiao@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>