optimisation: remove 1 more call to __divsf3

Changing (feedrate * feedmultiply) / 6000.f
to (feedrate * feedmultiply) * (1.f / 6000.f)

Replaces the call to __divsf3 with __mulsf3 which is better

No change in memory

Firmware/Marlin_main.cpp

@@ -9574,7 +9574,7 @@ void prepare_move(uint16_t start_segment_idx)
 void prepare_arc_move(bool isclockwise, uint16_t start_segment_idx) {
     float r = hypot(offset[X_AXIS], offset[Y_AXIS]); // Compute arc radius for mc_arc
     // Trace the arc
-    mc_arc(current_position, destination, offset, (feedrate * feedmultiply) / 6000.f, r, isclockwise, start_segment_idx);
+    mc_arc(current_position, destination, offset, (feedrate * feedmultiply) * (1.f / 6000.f), r, isclockwise, start_segment_idx);
     // As far as the parser is concerned, the position is now == target. In reality the
     // motion control system might still be processing the action and the real tool position
     // in any intermediate location.