s390/qeth: detach netdevice while card is offline

When a qeth card is offline, it has no connection to the HW. So none of
our control callbacks can run IO against it, and we can only cache the
input (eg a new MAC address) without providing proper feedback to the
caller. In this context, it seems much more reasonable to simply detach
the netdevice and let the kernel reject any interaction with it.

This also makes all sorts of internal state checks and locking obsolete.

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