pci/hotplug/pnv_php: Fix hotplug driver crash on Powernv

In the Linux kernel, the following vulnerability has been resolved:

pci/hotplug/pnv_php: Fix hotplug driver crash on Powernv

The hotplug driver for powerpc (pci/hotplug/pnv_php.c) causes a kernel

crash when we try to hot-unplug/disable the PCIe switch/bridge from

the PHB.

The crash occurs because although the MSI data structure has been

released during disable/hot-unplug path and it has been assigned

with NULL, still during unregistration the code was again trying to

explicitly disable the MSI which causes the NULL pointer dereference and

kernel crash.

The patch fixes the check during unregistration path to prevent invoking

pci_disable_msi/msix() since its data structure is already freed.