platforms/astbmc: Fix the PCI slot location code
platforms/astbmc: Fix the PCI slot location code

After commit aa928bfbd891 ("platforms/astbmc: Support PCI slot")
is merged, we have the assumption that PHB's base location code
is always valid. It's not true on openPower platforms, including
Garrison. It causes the PCI slot location code isn't exposed via
device-tree.

This fixes the above issue. The PCI slot location code contains
the label only if PHB's base location code is invalid.

Reported-by: Russell Currey <b5ebcef270cdc73fa0593440445c07a93ae1bccb@russell.cc>
Signed-off-by: Gavin Shan <0398910e4f4970259b451f0f9d0686cc5e961b81@linux.vnet.ibm.com>
Tested-by: Russell Currey <b5ebcef270cdc73fa0593440445c07a93ae1bccb@russell.cc>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

shenki/skiboot,legoater/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,qemu/skiboot,mikey/skiboot,stewart-ibm/skiboot,shenki/skiboot,qemu/skiboot,qemu/skiboot,stewart-ibm/skiboot,legoater/skiboot,mikey/skiboot,open-power/skiboot,qemu/skiboot,mikey/skiboot,open-power/skiboot,stewart-ibm/skiboot,qemu/skiboot,shenki/skiboot,shenki/skiboot,legoater/skiboot,open-power/skiboot,open-power/skiboot,legoater/skiboot