DDA, dda_start(): don't pass mb_tail_dda as parameter.

Instead, read the global variable directly.

The idea is that reading the global variable directly removes
the effort to build up a parameter stack, making things faster.

Actually, binary size increases by 4 bytes and the slowest step
takes 3 clock cycles longer. D'oh.

  ATmega sizes               '168   '328(P)   '644(P)     '1280
  Program:  19274 bytes      135%       63%       31%       15%
     Data:   2179 bytes      213%      107%       54%       27%
   EEPROM:     32 bytes        4%        2%        2%        1%

  short-moves.gcode statistics:
  LED on occurences: 888.
  LED on time minimum: 218 clock cycles.
  LED on time maximum: 398 clock cycles.
  LED on time average: 249.111 clock cycles.

  smooth-curves.gcode statistics:
  LED on occurences: 23648.
  LED on time minimum: 237 clock cycles.
  LED on time maximum: 441 clock cycles.
  LED on time average: 272.222 clock cycles.

  triangle-odd.gcode statistics:
  LED on occurences: 1636.
  LED on time minimum: 237 clock cycles.
  LED on time maximum: 398 clock cycles.
  LED on time average: 262.576 clock cycles.

dda.c

@@ -482,7 +482,6 @@ void dda_create(DDA *dda, const TARGET *target) {
 }
 
 /*! Start a prepared DDA
-	\param *dda pointer to entry in dda_queue to start
 
 	This function actually begins the move described by the passed DDA entry.
 
@@ -492,8 +491,8 @@ void dda_create(DDA *dda, const TARGET *target) {
 
 	Called both inside and outside of interrupts.
 */
-void dda_start(DDA *dda) {
-	// called from interrupt context: keep it simple!
+void dda_start() {
+  DDA *dda = mb_tail_dda;
 
   if (DEBUG_DDA && (debug_flags & DEBUG_DDA))
     sersendf_P(PSTR("Start: X %lq  Y %lq  Z %lq  F %lu\n"),

dda.h

@@ -187,7 +187,7 @@ void dda_new_startpoint(void);
 void dda_create(DDA *dda, const TARGET *target);
 
 // start a created DDA (called from timer interrupt)
-void dda_start(DDA *dda);
+void dda_start(void);
 
 // DDA takes one step (called from timer interrupt)
 void dda_step(DDA *dda);

dda_queue.c

@@ -71,7 +71,7 @@ void queue_step() {
     if (mb_tail != mb_head) {
       mb_tail = MB_NEXT(mb_tail);
       mb_tail_dda = &(movebuffer[mb_tail]);
-      dda_start(mb_tail_dda);
+      dda_start();
     }
     else {
       mb_tail_dda = NULL;
@@ -123,7 +123,7 @@ void enqueue_home(TARGET *t, uint8_t endstop_check, uint8_t endstop_stop_cond) {
       timer_reset();
       mb_tail = mb_head;  // Valid ONLY if the queue was empty before!
       mb_tail_dda = new_movebuffer; // Dito!
-      dda_start(mb_tail_dda);
+      dda_start();
       // Compensate for the cli() in timer_set().
       sei();
     }