MSI: Export the PCI_BUS_FLAGS_NO_MSI flag in sysfs

Export the PCI_BUS_FLAGS_NO_MSI flag of a PCI bus in the sysfs files
of its parent device and make it writable. Could be used to:
* disable MSI on a device which has not been blacklisted yet
* allow MSI when some setpci hacks enable MSI support (for instance
  on the ServerWorks HT2000 chipset where the MSI HT cap is disabled
  by default).
Architecture where some bus have no parent chipset cannot use this
strategy to change MSI support.

If the chipset does not have a subordinate bus, its 'bus_msi' file
is empty.

Also document and warn about the possible danger of changing the flag.

Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>