DDA: Move axis calculations into loops, part 6b.

Clean up code to reduce duplication by consolidating code into
loops for per-axis actions.

Part 6b moves do_step() from the "tidiest" place into where it's
currently used, dda.c. Binary size goes down another 34 bytes, to
a total savings of 408 bytes and performance is much better, but
still 16% lower than without using loops:

    SIZES             ATmega...  '168    '328(P)    '644(P)    '1280
    FLASH : 19874 bytes          139%       65%        32%       16%
    RAM   :  2302 bytes          225%      113%        57%       29%
    EEPROM:    32 bytes            4%        2%         2%        1%

short-moves.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 888.
Sum of all LED on time: 320000 clock cycles.
LED on time minimum: 351 clock cycles.
LED on time maximum: 772 clock cycles.
LED on time average: 360.36 clock cycles.

smooth-curves.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 9124.
Sum of all LED on time: 3875874 clock cycles.
LED on time minimum: 356 clock cycles.
LED on time maximum: 773 clock cycles.
LED on time average: 424.8 clock cycles.

triangle-odd.gcode
Statistics (assuming a 20 MHz clock):
LED on occurences: 1636.
Sum of all LED on time: 640357 clock cycles.
LED on time minimum: 351 clock cycles.
LED on time maximum: 773 clock cycles.
LED on time average: 391.416 clock cycles.

dda.c

@@ -471,6 +471,18 @@ void dda_start(DDA *dda) {
 	current_position.F = dda->endpoint.F;
 }
 
+// step the 'n' axis
+static void do_step(enum axis_e n) {
+  if (n == X)
+    x_step();
+  else if (n == Y)
+    y_step();
+  else if (n == Z)
+    z_step();
+  else if (n == E)
+    e_step();
+}
+
 /*! STEP
 	\param *dda the current move
 

pinio.c

@@ -42,16 +42,3 @@ void power_off() {
 
 	ps_is_on = 0;
 }
-
-// step the 'n' axis
-void do_step(enum axis_e n) {
-  if (n == X)
-    x_step();
-  else if (n == Y)
-    y_step();
-  else if (n == Z)
-    z_step();
-  else if (n == E)
-    e_step();
-}
-

pinio.h

@@ -6,7 +6,6 @@
 #define	_PINIO_H
 
 #include	"config_wrapper.h"
-#include "dda.h"
 
 #ifdef SIMULATOR
   #include "simulator.h"
@@ -34,8 +33,6 @@ inline void power_init(void) {
 void power_on(void);
 void power_off(void);
 
-void do_step(enum axis_e n);
-
 /*
 X Stepper
 */