net: stmmac: xgmac: Only get SPH header len if available

Split Header length is only available when L34T == 0. Fix this by
correctly checking if L34T is zero before trying to get Header length.

Fixes: 67afd6d1 ("net: stmmac: Add Split Header support and enable it in XGMAC cores")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>