export pcie_flr and remove copies of it in drivers V2

[Christoph Hellwig]

Hi all,

this exports the PCI layer pcie_flr helper, and removes various opencoded
copies of it.

Changes since V1:
 - rebase on top of the pci/virtualization branch
 - fixed the probe case in __pci_dev_reset
 - added ACKs from Bjorn