dda_queue.c: inline a simplified version of next_move().

As we're in an interrupt already, we can simplify the test for an
empty queue. Slowest step down to 446 clock cycles, another 26
ticks less. Binary size only 36 bytes up:

  ATmega sizes               '168   '328(P)   '644(P)     '1280
  Program:  19472 bytes      136%       64%       31%       16%
     Data:   2177 bytes      213%      107%       54%       27%
   EEPROM:     32 bytes        4%        2%        2%        1%

  short-moves.gcode statistics:
  LED on occurences: 888.
  LED on time minimum: 226 clock cycles.
  LED on time maximum: 403 clock cycles.
  LED on time average: 262.922 clock cycles.

  smooth-curves.gcode statistics:
  LED on occurences: 23648.
  LED on time minimum: 251 clock cycles.
  LED on time maximum: 446 clock cycles.
  LED on time average: 286.203 clock cycles.

  triangle-odd.gcode statistics:
  LED on occurences: 1636.
  LED on time minimum: 251 clock cycles.
  LED on time maximum: 403 clock cycles.
  LED on time average: 276.561 clock cycles.

dda_queue.c

@@ -83,7 +83,17 @@ void queue_step() {
 
   // Start the next move if this one is done.
   if ( ! movebuffer[mb_tail].live) {
-		next_move();
+    /**
+      This is a simplified version of next_move() (which we'd use it it wasn't
+      so performance critical here).
+
+      queue_empty() used in next_move() needs no atomic protection, because
+      we're in an interrupt already.
+    */
+    if (mb_tail != mb_head) {
+      mb_tail = MB_NEXT(mb_tail);
+      dda_start(&movebuffer[mb_tail]);
+    }
   }
 }