PCI/ATS: Disable PF/VF ATS service independently

Previously we didn't disable the PF ATS until all associated VFs had
disabled it.  But per PCIe spec r5.0, sec 9.3.7.8, the ATS Capability in
VFs and associated PFs may be enabled independently.  Leaving ATS enabled
in the PF unnecessarily may have power and performance impacts.

Remove this dependency logic in the ATS enable/disable code.

[bhelgaas: commit log]
Suggested-by: Ashok Raj <ashok.raj@intel.com>
Link: https://lore.kernel.org/r/8163ab8fa66afd2cba514ae95d29ab12104781aa.1567029860.git.sathyanarayanan.kuppuswamy@linux.intel.com
Link: https://lore.kernel.org/r/20190905193146.90250-4-helgaas@kernel.org
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Keith Busch <keith.busch@intel.com>