PCI/DPC: Use the generic pcie_do_fatal_recovery() path

Our goal is to handle ERR_FATAL errors similarly, whether they are reported
via AER or via DPC.  A previous commit changed AER so it handles ERR_FATAL
by calling driver .remove() methods and resetting the Link.  DPC already
does that (although the Link reset is done automatically by hardware and
happens before we call the driver .remove() methods).

Restructure the DPC code so it calls the same pcie_do_fatal_recovery()
interface used by AER.  This makes it clearer that we want to use the same
path.

Implement the .reset_link() method used by pcie_do_fatal_recovery().  For
DPC, the actual reset is done automatically by hardware, so we really only
have to wait for the Link to be inactive, then release the Port from DPC.

Signed-off-by: Oza Pawandeep <poza@codeaurora.org>
[bhelgaas: changelog, DPC_FATAL is not a bitfield, can be sequential]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>