PCI: Fix BUG on device attach failure

Previously when pci_bus_add_device() called device_attach() and it returned
a negative value, we emitted a WARN but carried on.

Commit ab1a187b ("PCI: Check device_attach() return value always"),
introduced in Linux 4.6-rc1, changed this to unwind all steps preceding
device_attach() and to not set dev->is_added = 1.

The latter leads to a BUG if pci_bus_add_device() was called from
pci_bus_add_devices().  Fix by not recursing to a child bus if
device_attach() failed for the bridge leading to it.

This can be triggered by plugging in a PCI device (e.g. Thunderbolt) while
the system is asleep.  The system locks up when woken because
device_attach() returns -EPROBE_DEFER.

Fixes: ab1a187b

 ("PCI: Check device_attach() return value always")
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>