net/mlx5e: Only allow offloading decap egress (egdev) flows

We get egress rules through the egdev mechanism when the ingress device
is not supporting offload, with the expected use-case of tunnel decap
ingress rule set on shared tunnel device.

Make sure to offload egress/egdev rules only if decap action (tunnel key
unset) exists there and err otherwise.

Fixes: 717503b9 ("net: sched: convert cls_flower->egress_dev users to tc_setup_cb_egdev infra")
Signed-off-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>