dda_queue.c: take advantage of a special case.

No functional change or stepping performance improvement, but a
14 bytes smaller binary:

  ATmega sizes               '168   '328(P)   '644(P)     '1280
  Program:  19426 bytes      136%       64%       31%       16%
     Data:   2177 bytes      213%      107%       54%       27%
   EEPROM:     32 bytes        4%        2%        2%        1%

dda_queue.c

@@ -139,7 +139,7 @@ void enqueue_home(TARGET *t, uint8_t endstop_check, uint8_t endstop_stop_cond) {
         version is inlined (and simplified) in queue_step().
       */
       timer_reset();
-      mb_tail = MB_NEXT(mb_tail);
+      mb_tail = mb_head;  // Valid ONLY if the queue was empty before!
       dda_start(&movebuffer[mb_tail]);
       // Compensate for the cli() in timer_set().
       sei();