PCI: dwc: Convert to use pci_host_probe()

Now that there are no more .scan_bus() callbacks, we can remove it and just
use pci_host_probe().

Link: https://lore.kernel.org/r/20200821035420.380495-17-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Jingoo Han <jingoohan1@gmail.com>
Cc: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>