dm: pci: Fix device PCI iteration

When there is no PCI bus uclass_first_device will return no bus and no
error which will result in pci_find_first_device calling
skip_to_next_device with no bus, and the bus is only checked at the end
of the while cycle, not the beginning.

Fixes: 76c3fbcd ("dm: pci: Add a way to iterate through all PCI devices")
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>