powerpc/powernv/pci: Check status of a PHB before using it

If the firmware encounters an error (internal or HW) during initialization
of a PHB, it might leave the device-node in the tree but mark it disabled
using the "status" property. We should check it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>