pci: Fix compiler warnings in dm_pciauto_setup_device()

Fix the following compiler warnings when DEBUG is on.

warning: 'bar_res' may be used uninitialized in this function.
drivers/pci/pci_auto.c:101:21:
   if (!enum_only && pciauto_region_allocate(bar_res, bar_size,
                        ^

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