powerpc/eeh: Enhance converting EEH dev

Under some special circumstances, the EEH device doesn't have the
associated device tree node or PCI device. The patch enhances those
functions converting EEH device to device tree node or PCI device
accordingly to avoid unnecessary system crash.

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