s390/qeth: defer IPv6 address notifier events

The inet6addr_chain is atomic. So instead of starting the cmd IO for
SETIP / DELIP straight from the notifier callback, run it from a
workqueue. This is the last step towards removal of cmd IO completion
polling.

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