s390/qeth: be drop monitor friendly

As part of the TX completion path, qeth_release_skbs() frees the completed
skbs with __skb_queue_purge(). This ends in kfree_skb(), reporting every
completed skb as dropped.
On the other hand when dropping an skb in .ndo_start_xmit, we end up
calling consume_skb()... where we should be using kfree_skb() so that
drop monitors get notified.

Switch the drop/consume logic around, and also don't accumulate dropped
packets in the tx_errors statistics.

Fixes: dc149e37

 ("s390/qeth: replace open-coded skb_queue_walk()")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>