Once the call to qdio_establish() has completed, qdio is free to deliver
data IRQs to the device driver's IRQ poll handler.

For qeth (the only qdio driver that currently uses IRQ polling) this is
problematic, since the IRQs can arrive before its NAPI instance is
even registered. Calling napi_schedule() from qeth_qdio_start_poll()
then crashes in various nasty ways.

Until recently qeth checked for IFF_UP to drop such early interrupts,
but that's fragile as well since it doesn't enforce any ordering.

Fix this properly by bringing up the qdio device in IRQS_DISABLED mode,
and have the driver explicitly opt-in to receive data IRQs.
qeth does so from qeth_open(), which kick-starts a NAPI poll and then
calls qdio_start_irq() from qeth_poll().

Also add a matching qdio_stop_irq() in qeth_stop() to switch the qdio
dataplane back into a disabled state.

Fixes: 3d35dbe6 ("s390/qeth: don't check for IFF_UP when scheduling napi")
CC: Qian Cai <cai@lca.pw>
Reported-by: Qian Cai <cai@lca.pw>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>