s390/qeth: don't keep statistics for tx timeout

struct netdev_queue contains a counter for tx timeouts, which gets
updated by dev_watchdog(). So let's not attempt to maintain our own
statistics, in particular not by overloading the skb-error counter.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>