dda.c: simplify copy of startpoint.

This reduces binary size by 26 bytes without drawback.

  ATmega sizes               '168   '328(P)   '644(P)     '1280
  Program:  19526 bytes      137%       64%       31%       16%
     Data:   2175 bytes      213%      107%       54%       27%
   EEPROM:     32 bytes        4%        2%        2%        1%

dda.c

@@ -974,9 +974,6 @@ void update_current_position() {
     current_position.F = dda->endpoint.F;
 	}
   else {
-    for (i = X; i < AXIS_COUNT; i++) {
-      current_position.axis[i] = startpoint.axis[i];
-    }
-    current_position.F = startpoint.F;
+    memcpy(&current_position, &startpoint, sizeof(TARGET));
 	}
 }