sunvnet: failed trigger should not cause BUG_ON()

An error return from __vnet_tx_trigger() sets the TX descriptor to
VIO_DESC_FREE while leaving port->tx_bufs[txi].skb set. This leads
to a BUG_ON() the next time this descriptor is used.

This patch frees the pending skb when getting a trigger error to
match the VIO_DESC_FREE state.

Signed-off-by: David L Stevens <david.stevens@oracle.com>
Acked-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>