pci: pci-uclass: Make DT subnode parse optional

Parse subnode DT properties only if parent node is valid.
Otherwise, assert is triggered on ofnode_valid in ofnode_first_subnode
from dev_for_each_subnode.

Signed-off-by: Suneel Garapati <sgarapati@marvell.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>