qeth: start dev queue after tx drop error

In case the xmit function drop out with an error, we have to wake
the netdevice queue to start another xmit.

Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>