PCI: fix upstream P2P bridge checks when enabling OBFF and LTR
pci_enable_obff() and pci_enable_ltr() incorrectly check "dev->bus" instead
of "dev->bus->self" to determine whether the upstream device is a P2P
bridge or a host bridge. For devices on the root bus, the upstream device
is a host bridge, "dev->bus != NULL" and "dev->bus->self == NULL", and we
panic with a null pointer dereference.
These functions should previously have panicked when called on devices
supporting OBFF or LTR, so they should be regarded as untested.