PCI/MSI: Add "msi_bus" sysfs MSI/MSI-X control for endpoints

The "msi_bus" sysfs file for bridges sets a bus flag to allow or disallow
future driver requests for MSI or MSI-X.  Previously, the sysfs file
existed for endpoints but did nothing.

Add "msi_bus" support for endpoints, so an administrator can prevent the
use of MSI and MSI-X for individual devices.

Note that as for bridges, these changes only affect future driver requests
for MSI or MSI-X, so drivers may need to be reloaded.

Add documentation for the "msi_bus" sysfs file.

[bhelgaas: changelog, comments, add "subordinate", add endpoint printk,
rework bus_flags setting, make bus_flags printk unconditional]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>