Turn off led when exiting the app

Right now the led stays turned on after I exit the app. 

I think adding

```furi_hal_light_set(LightRed | LightGreen | LightBlue, 0);```

around

https://github.com/arha/flipper-dcf77/blob/588bcf7e87db5061090c8c3e23e0e50c6e4d347b/dcf77_app.c#L434

Would do it.

PS: It didn't work with my **Auriol AFT 77 A1** sadly.