dm: pci: Fix pci_last_busno() to return the real last bus no

Currently pci_last_busno() only checks the last bridge device
under the first UCLASS_PCI device. This is not the case when
there are multiple bridge devices.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>