nitrokey: Allow transitioning from runtime to DFU mode
nitrokey: Allow transitioning from runtime to DFU mode

This allows the Nitrokey (and indeed, any device that has to use two plugins)
to be updated using fwupd and gnome-software.

superm1/fwupd,superm1/fwupd,hughsie/fwupd,vathpela/fwupd,hughsie/fwupd,vathpela/fwupd,hughsie/fwupd,vathpela/fwupd,fwupd/fwupd,fwupd/fwupd,superm1/fwupd,vathpela/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd