PCI/PM: Set power.strict_midlayer in pci_pm_init()

The PCI bus type does not expect its runtime PM suspend callback
function, pci_pm_runtime_suspend(), to be invoked at all during system-
wide suspend and resume, and it does not expect its runtime resume
callback function, pci_pm_runtime_resume(), to be invoked at any point
when runtime PM is disabled for the given device during system-wide
suspend and resume, so make it express that expectation by setting
power.strict_midlayer for all PCI devices in pci_pm_prepare() and
clear it in pci_pm_complete().

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/1925097.atdPhlSkOF@rjwysocki.net