Would be nice to know which version/commit of OpenWrt you are running.
Also, I think the words driver and firmware are wrongly placed here. I assume you have not built a firmware, but you probably have built "OpenWrt, which includes the firmware, the driver and a bunch of other stuff". I would rename this thread from Driver mt7915e to "Package loss - Zyxel box Multi M1 (MT7915e)" or something similar.
Also, obviously the fault could also lie with your Livebox 6 or your PC or your Orange TV. the OpenWrt relayd device is only 1 out of 3 or 4 devices in your network.
Also, how did you know that you have package loss?