PCI: fix truncation of resource size to 32 bits

_pci_assign_resource() took an int "size" argument, which meant that
sizes larger than 4GB were truncated.  Change type to resource_size_t.

[bhelgaas: changelog]
Signed-off-by: Nikhil P Rao <nikhil.rao@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>