If the volume down key has stopped working as intended and the volume down key keeps being triggered, I am afraid that no matter how many times you restart your phone, the phone will remain in safe mode. This due to the fact that safe mode is triggered by holding the volume down key while during start up.
In this case I suggest that you get in contact with your local support team. They can help you in getting your phone sent to an authorized service center in order to be examined and repaired. If you have any questions regarding the service, such as routines, warranty and consumer laws, these type of questions are best answered by your local support team as well, as they vary between markets.
I hope that this answered your question. Feel free to return if you have any further questions!