synaptics-mst: Allow MST to fall back if no PCI device is marked as parent
synaptics-mst: Allow MST to fall back if no PCI device is marked as parent

At the moment not having a udev parent was causing the dock device to not be
detected.

hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd