RSS, when enabled, will bypass the L3 and L4 filtering causing it not
to work. Add a check before trying to setup the filters.

Fixes: 425eabdd ("net: stmmac: Implement L3/L4 Filters using TC Flower")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>