trivial: dell-dock: Lower MST communication speed to 400 KHz
trivial: dell-dock: Lower MST communication speed to 400 KHz

It was identified that at least one other device on this shared
I2C bus can't operate at the speed.  This means that it may
inadvertently respond to commands intended for another device.

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