MFC r197099: pci(4): don't perform maximum register number check

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
Approved by: re (kib)