net: stmmac: Fix TSO descriptor with Enhanced Addressing

When using addressing > 32 bits the TSO first descriptor only has the
header so we can't set the payload field for this descriptor. Let's
reset the variable so that buffer 2 value is zero.

Fixes: a993db88 ("net: stmmac: Enable support for > 32 Bits addressing in XGMAC")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>