trivial: dell-dock: Save the base type in private structure
trivial: dell-dock: Save the base type in private structure

This may be used in the future for other expansion and determination.

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