Wait until the QDIO data connection is severed. Otherwise the device
might still be processing the buffers, and end up accessing skb data
that we already freed.

Fixes: 8b5026bc ("s390/qeth: fix qdio teardown after early init error")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>