stepper: Default to a high direction pin meaning positive direction

Invert the default meaning of the stepper direction pin.  Instead of
treating a low value as position motion, treat a high value as
positive motion.  This matches what other firmwares do, and it matches
what common stepper motor drivers document.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/makergear-m2-2012.cfg

@@ -5,7 +5,7 @@
 
 [stepper_x]
 step_pin: PC0
-dir_pin: PL1
+dir_pin: !PL1
 enable_pin: !PA7
 step_distance: .0225
 max_velocity: 500
@@ -20,7 +20,7 @@ position_max: 200
 
 [stepper_y]
 step_pin: PC1
-dir_pin: !PL0
+dir_pin: PL0
 enable_pin: !PA6
 step_distance: .0225
 max_velocity: 500
@@ -35,7 +35,7 @@ position_max: 250
 
 [stepper_z]
 step_pin: PC2
-dir_pin: PL2
+dir_pin: !PL2
 enable_pin: !PA5
 step_distance: .005
 max_velocity: 250
@@ -51,7 +51,7 @@ position_max: 200
 
 [extruder]
 step_pin: PC3
-dir_pin: !PL6
+dir_pin: PL6
 enable_pin: !PA4
 step_distance: .004242
 max_velocity: 200000