OpenFirmware marks devices as failed in the device-tree when a hardware
problem is detected.  The kernel needs to fail config reads/writes to
prevent a kernel crash when incorrect data is read.

This patch validates that the device-node is not marked "fail" when
config space reads/writes are attempted.
Signed-off-by: Jake Moilanen <moilanen@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>