net: stmmac: fix NULL pointer dereference in stmmac_get_tx_hwtstamp

When timestamping is enabled, stmmac_tx_clean will call
stmmac_get_tx_hwtstamp to get tx TS.
But the skb can be NULL because the last of its tx_skbuff is NULL
if this packet frame is filled in more than one descriptors.

To fix the issue, change the code:
- Store TX skb to the tx_skbuff[] of frame's last segment.
- Check skb is not NULL in stmmac_get_tx_hwtstamp.

Signed-off-by: Bruce Liu <damuzi000@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>