pci: Avoid a crash in device_is_on_pci_bus()

This function cannot currently be called on the root node. Add a check
for this as well as a test.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>