net: ethernet: stmmac: GMAC4.xx: Fix TX descriptor preparation

On GMAC4.xx each descriptor contains 2 buffers of 16KB (each).
Initially, those 2 buffers was filled in dwmac4_rd_prepare_tx_desc but
it is actually not needed. Indeed, stmmac driver supports frame up to
9000 bytes (jumbo). So only one buffer is needed.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>