Init Serial on RELAY_PROVIDER_STM (#1130)

code/espurna/system.ino

@@ -140,7 +140,7 @@ void _systemSetupSpecificHardware() {
 
     // These devices use the hardware UART
     // to communicate to secondary microcontrollers
-    #if defined(ITEAD_SONOFF_RFBRIDGE) || defined(ITEAD_SONOFF_DUAL) || defined(STM_RELAY)
+    #if defined(ITEAD_SONOFF_RFBRIDGE) || defined(ITEAD_SONOFF_DUAL) || (RELAY_PROVIDER == RELAY_PROVIDER_STM)
         Serial.begin(SERIAL_BAUDRATE);
     #endif