LinuxKPI: pci.h make pci_dev argument const for pci_{read,write}_config*()

Make the struct pci_dev argument to the pci_{read,write}_config*()
functions "const" to match the Linux definition as some drivers
try to pass in a const argument which we currently fail to honor.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D32644