PCI: Wait for pending transactions to complete before 82599 FLR

Before initiating an FLR, we should wait for completion of any outstanding
non-posted requests.  See PCIe spec r3.0, sec 6.6.2.

This makes reset_intel_82599_sfp_virtfn() very similar to the generic
pcie_flr().  The only difference is that the 82599 doesn't report FLR
support in the VF Device Capability register.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>