Various fixes for PCI _OSC handling so HotPlug works again.

• Rename the default implementation of 'pcib_request_feature' and add a pcib_request_feature() wrapper function (as is often done for new-bus APIs implemented via kobj) that accepts a single function. Previously the call to pcib_request_feature() ended up invoking the method on the great-great-grandparent of the bridge device instead of the grandparent. For a bridge that was a direct child of pci0 on x86 this resulted in the method skipping over the Host-PCI bridge driver and being invoked against nexus0
• When invoking _OSC from a Host-PCI bridge driver, invoke device_get_softc() against the Host-PCI bridge device instead of the child bridge that is requesting HotPlug. Using the wrong softc data resulted in garbage being passed for the ACPI handle causing the _OSC call to fail.
• While here, perform some other cleanups to _OSC handling in the ACPI Host-PCI bridge driver:
  • Don't invoke _OSC when requesting a control that has already been granted by the firmware.
  • Don't set the first word of the capability array before invoking _OSC. This word is always set explicitly by acpi_EvaluateOSC() since it is UUID-independent.
  • Don't modify the set of granted controls unless _OSC doesn't exist (which is treated as always successful), or the _OSC method doesn't fail.
  • Don't require an _OSC status of 0 for success. _OSC always returns the updated control mask even if it returns a non-zero status in the first word.
  • Whine if _OSC ever tries to revoke a previously-granted control. (It is not supposed to do that.)
• While here, add constants for the _OSC status word in acpivar.h (though currently unused).

Reported by: adrian
Reviewed by: imp
MFC after: 1 week
Tested on: Lenovo x220
Differential Revision: https://reviews.freebsd.org/D10520