s390/qeth: only handle IRQs while device is online

A qeth device that's offline should not be receiving any IRQs - all
pending IOs have been terminated, and we avoid starting any new ones.

So rather than immediately registering the IRQ handler when the device
is probed, only register it while the device is online.

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