Spindle PWM update for Mega2560

- Spindle PWM is set with a 16-bit value, rather than 8-bit on a 328p.
Updated stepper.c to reflect this change.

doc/log/commit_log_v1.1.txt

@@ -0,0 +1,13 @@
+----------------
+Date: 2016-12-19
+Author: Sonny Jeon
+Subject: Grbl v1.1e port to Mega2560 branch
+
+- v1.1e
+
+- Ported all current changes from the main Grbl 328p repo to the
+Mega2560 branch. Main differences are increased planner, RX, TX, and
+line buffers. And an optional sleep timeout feature.
+
+- WARNING: Code compiles, but has not been tested.
+

grbl/grbl.h

@@ -23,7 +23,7 @@
 
 // Grbl versioning system
 #define GRBL_VERSION "1.1e"
-#define GRBL_VERSION_BUILD "20161219"
+#define GRBL_VERSION_BUILD "20161219.2"
 
 // Define standard libraries used by Grbl.
 #include <avr/io.h>

grbl/stepper.c

@@ -78,7 +78,7 @@ typedef struct {
   #else
     uint8_t prescaler;      // Without AMASS, a prescaler is required to adjust for slow timing.
   #endif
-  uint8_t spindle_pwm;
+  uint16_t spindle_pwm;
 } segment_t;
 static segment_t segment_buffer[SEGMENT_BUFFER_SIZE];
 
@@ -152,7 +152,7 @@ typedef struct {
   float decelerate_after; // Deceleration ramp start measured from end of block (mm)
 
   float inv_rate;    // Used by PWM laser mode to speed up segment calculations.
-  uint8_t current_spindle_pwm; 
+  uint16_t current_spindle_pwm; 
 } st_prep_t;
 static st_prep_t prep;