PCI/PME: Fix possible use-after-free on remove

In remove(), ensure that the PME work cannot run after kfree() is called.
Otherwise, this could result in a use-after-free.

This issue was detected with the help of Coccinelle.

Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Sinan Kaya <okaya@kernel.org>
Cc: Frederick Lawler <fred@fredlawl.com>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Keith Busch <keith.busch@intel.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>