PCI: Separate pci_bus_read_dev_vendor_id from pci_scan_device

We can reuse it for pciehp probing.

-v2: according to Kenji, fix crs timeout checking, and export the function
     for later use when pciehp is compiled as a module.

Suggested-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>