nfp: flower: fix unsupported pre_tunnel flows

There are some pre_tunnel flows combinations which are incorrectly being
offloaded without proper support, fix these.

- Matching on MPLS is not supported for pre_tun.
- Match on IPv4/IPv6 layer must be present.
- Destination MAC address must match pre_tun.dev MAC

Fixes: 120ffd84 ("nfp: flower: verify pre-tunnel rules")
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>