pci: Reconcile MPS before attaching PCIe devices

Reconcile each newly enumerated link as a unit before child drivers
attach. Firmware may leave Bus Master Enable set after handoff, so use
the bus attachment state rather than that bit to identify the cold
phase.

Preserve an established hierarchy during rescan and hot-add. Refuse a
reduction below a switch because recursive enumeration may already have
made a sibling subtree live; lowering only the local port or Root Port
would produce an inconsistent path. Report capability and active-use
conflicts distinctly.

Handle OFW PCI buses that clone the generic enumeration path.

MFC after:	2 weeks

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