synapticsmst: Fix const warnings on Centos7
synapticsmst: Fix const warnings on Centos7

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