spapr_pci: Fix config space corruption

When disabling MSI/MSIX via "ibm,change-msi" RTAS call, no check was made
if MSI or MSIX is actually supported and the MSI message was reset
unconditionally. If this happened on a device which does not support MSI
(but does support MSIX, otherwise "ibm,change-msi" would not be called),
this device would have PCIDevice::msi_cap field (MSI capability offset)
set to zero and writing a vector would actually clear PCI status.

This clears MSI message only if MSI or MSIX is present on a device.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
