pci: Abort early if bus does not exist

When listing the devices on a PCI bus, the current code will blindly try
to access all devices. Internally this causes pci_bus_to_hose() to be
repeatedly called and output an error message every time. Prevent this
by calling pci_bus_to_hose() once and abort early if no bus was found.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Simon Glass <sjg@chromium.org>