dfu: Fix a trivial unused variable spotted by clang
dfu: Fix a trivial unused variable spotted by clang

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