ibmveth: lost IRQ while closing/opening device leads to service loss

The order of freeing the IRQ and freeing the device in firmware
in ibmveth_close can cause the adapter to become unusable after a
subsequent ibmveth_open.  Only a reboot of the OS will make the
network device usable again. This is seen when cycling the adapter
up and down while there is network activity.

There is a window where an IRQ will be left unserviced (H_EOI will not
be called).  The solution is to make a VIO_IRQ_DISABLE h_call, free the
device with firmware, and then call free_irq.

Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>