Add COUNT macro and apply

Marlin/digipot_mcp4451.cpp

@@ -50,7 +50,7 @@ void digipot_i2c_init() {
   const float digipot_motor_current[] = DIGIPOT_I2C_MOTOR_CURRENTS;
   Wire.begin();
   // setup initial currents as defined in Configuration_adv.h
-  for(int i = 0; i <= sizeof(digipot_motor_current) / sizeof(float); i++) {
+  for(int i = 0; i < COUNT(digipot_motor_current); i++) {
     digipot_i2c_set_current(i, digipot_motor_current[i]);
   }
 }