net: stmmac: selftests: Fix L3/L4 Filtering test

Since commit 319a1d19, stmmac only support basic HW stats type for
action. Set this field in the L3/L4 Filtering test so that it correctly
setups the filter instead of returning EOPNOTSUPP.

Fixes: 319a1d19

 ("flow_offload: check for basic action hw stats type")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>