linux: Fix platform device PCI detection for complex bus topologies

Suppose you're in a Hyper-V guest and are trying to use PCI passthrough.
The ID_PATH that udev will construct for that looks something like
"acpi-VMBUS:00-pci-b8c8:00:00.0", and obviously looking for "pci-" in
the first four characters of that is going to not work.

Instead, strstr. I suppose it's possible you could have _multiple_ PCI
buses in the path, in which case you'd want strrstr, if that were a
thing.
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