Different sub-kinds of PCI buses may have different rules and
thus it is up for the bus backends to do proper input checks.
For example, PCIe allows configuration register numbers < 0x1000,
while for PCI proper the limit is 0x100.
And, in fact, the buses already do the checks.
Reviewed by: jhb
MFC after: 1 week
X-ToDo: add check for negative value to bus backends
X-ToDo: use named constant for maximum PCIe register
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