PCI: disable Bus Master on PCI device shutdown

Disable Bus Master bit on the device in pci_device_shutdown() to ensure PCI
devices do not continue to DMA data after shutdown.  This can cause memory
corruption in case of a kexec where the current kernel shuts down and
transfers control to a new kernel while a PCI device continues to DMA to
memory that does not belong to it any more in the new kernel.

I have tested this code on two laptops, two workstations and a 16-socket
server.  kexec worked correctly on all of them.

Signed-off-by: Khalid Aziz <khalid.aziz@hp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>