drivers/of: Fix depth for sub-tree blob in unflatten_dt_nodes()

drivers/of: Fix depth for sub-tree blob in unflatten_dt_nodes()

The function is unflattening device sub-tree blob if @dad passed to
the function is valid. Currently, this functionality is used by PPC
PowerNV PCI hotplug driver only. There are possibly multiple nodes
in the first level of depth, fdt_next_node() bails immediately when
@depth becomes negative before the second device node can be probed
successfully. It leads to the device nodes except the first one won't
be unflattened successfully.

This fixes the issue by setting the initial depth (@inital_depth) to
1 when this function is called to unflatten device sub-tree blob. No
logic changes when this function is used to unflatten non-sub-tree
blob.

Cc: Rhyland Klein <rklein@nvidia.com>
Fixes: 78c44d910 ("drivers/of: Fix depth when unflattening devicetree")
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Tested-by: Rhyland Klein <rklein@nvidia.com>
Tested-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Rob Herring <robh@kernel.org>