PCI: Disable Samsung SM961/PM961 NVMe before FLR

The Samsung SM961/PM961 (960 EVO) sometimes fails to return from FLR with
the PCI config space reading back as -1.  A reproducible instance of this
behavior is resolved by clearing the enable bit in the NVMe configuration
register and waiting for the ready status to clear (disabling the NVMe
controller) prior to FLR.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1542494


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>