Refactor riverflow RGB matrix loop (#24203)

Check for LED flags at the start of loop to skip early

quantum/rgb_matrix/animations/riverflow_anim.h

@@ -7,11 +7,11 @@ RGB_MATRIX_EFFECT(RIVERFLOW)
 bool RIVERFLOW(effect_params_t* params) {
     RGB_MATRIX_USE_LIMITS(led_min, led_max);
     for (uint8_t i = led_min; i < led_max; i++) {
+        RGB_MATRIX_TEST_LED_FLAGS();
         HSV      hsv  = rgb_matrix_config.hsv;
         uint16_t time = scale16by8(g_rgb_timer + (i * 315), rgb_matrix_config.speed / 8);
         hsv.v         = scale8(abs8(sin8(time) - 128) * 2, hsv.v);
         RGB rgb       = rgb_matrix_hsv_to_rgb(hsv);
-        RGB_MATRIX_TEST_LED_FLAGS();
         rgb_matrix_set_color(i, rgb.r, rgb.g, rgb.b);
     }