fix speed calculation. should fix low voltage poweroff while driving

Src/main.c

@@ -408,7 +408,7 @@ int main(void) {
     speedL = speedLeftFixdt >> 4;  // convert fixed-point to integer
     speedR = speedRightFixdt >> 4;  // convert fixed-point to integer
 
-    speed = (speedL+speedR)/2;
+    speed = (abs(speedL)+abs(speedR))/2;
 
     // ####### MIXER #######
     // speedR = CLAMP((int)(speed * SPEED_COEFFICIENT -  steer * STEER_COEFFICIENT), -1000, 1000);