tx_skbuff is initialized to NULL in init_dma_tx_desc_rings(), which is
called from ndo_open().

stmmac_tx_clean() frees any non-NULL skb, and sets the tx_skbuff
entry to NULL. Hence, there is no need to set skbuff entries to NULL
in stmmac_xmit()/stmmac_tso_xmit(), and doing so falsely gives the
reader the impression that it is needed.
Do not clear tx_skbuff entries in stmmac_xmit()/stmmac_tso_xmit().
Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>