powerpc/eeh: Make EEH driver device hotplug safe

In the Linux kernel, the following vulnerability has been resolved:

powerpc/eeh: Make EEH driver device hotplug safe

Multiple race conditions existed between the PCIe hotplug driver and the

EEH driver, leading to a variety of kernel oopses of the same general

nature:

<pcie device unplug>

<eeh driver trigger>

<hotplug removal trigger>

<pcie tree reconfiguration>

<eeh recovery next step>

<oops in EEH driver bus iteration loop>

A second class of oops is also seen when the underlying bus disappears

during device recovery.

Refactor the EEH module to be PCI rescan and remove safe. Also clean

up a few minor formatting / readability issues.