unicore32/PCI: Remove redundant call of pci_bus_add_devices()

pci_scan_bus() has called pci_bus_add_devices() already, so remove the
redundant call of pci_bus_add_devices().  subsys_init() callbacks will be
invoked before device_init() callbacks, so it should be safe to remove the
redundant calls.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>