PCI/PME: Restructure pcie_pme_suspend() to prevent compiler warning

Previously we had this:

  if (wakeup)
    ret = enable_irq_wake(...);
  if (!wakeup || ret)
    ...

"ret" is only evaluated when "wakeup" is true, and it is always initialized
in that case, but gcc isn't smart enough to figure that out and warns:

  drivers/pci/pcie/pme.c:414:14: warning: 'ret' may be used uninitialized in this function [-Wmaybe-uninitialized]

Restructure the code slightly to make it easier for gcc (and maybe for
humans as well).

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>