powerpc/eeh: Remove reference to PCI device

We will rely on pcibios_release_device() to remove the EEH cache
and unbind EEH device for the specific PCI device. So we shouldn't
hold the reference to the PCI device from EEH cache and EEH device.
Otherwise, pcibios_release_device() won't be called as we expected.
The patch removes the reference to the PCI device in EEH core.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>