Fix mcp4728_setVref_all as followup to #4967

Marlin/dac_mcp4728.cpp

  */
 uint8_t mcp4728_setVref_all(uint8_t value) {
   Wire.beginTransmission(DAC_DEV_ADDRESS);
-  Wire.write(GAINWRITE | (value ? 0x0F : 0x00));
+  Wire.write(VREFWRITE | (value ? 0x0F : 0x00));
   return Wire.endTransmission();
 }
 /**