PCI: loongson: Don't access non-existent devices

On LS2K/LS7A, config reads to some non-existent devices don't return
0xffffffff (they are hidden devices for debug, and accessing the config
space may cause machine hang). This is a hardware flaw but we can only
avoid it by software now.

Link: https://lore.kernel.org/r/20220714124216.1489304-5-chenhuacai@loongson.cn


Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>