amd-xgbe: Associate Tx SKB with proper ring descriptor

The SKB for a Tx packet is associated with an xgbe_ring_data structure
in the xgbe_map_tx_skb function.  However, it is being saved in the
structure after the last structure used when the SKB is mapped.  Use
the last used structure to save the SKB value.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>