mlx5: Don't accidentally set RTO_ONLINK before mlx5e_route_lookup_ipv4_get()

Mask the ECN bits before calling mlx5e_route_lookup_ipv4_get(). The
tunnel key might have the last ECN bit set. This interferes with the
route lookup process as ip_route_output_key_hash() interpretes this bit
specially (to restrict the route scope).

Found by code inspection, compile tested only.

Fixes: c7b9038d ("net/mlx5e: TC preparation refactoring for routing update event")
Fixes: 9a941117

 ("net/mlx5e: Maximize ip tunnel key usage on the TC offloading path")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>