Fix SERIAL_PORT_2 sanity check

Marlin/src/inc/SanityCheck.h

   #error "Set SERIAL_PORT to the port on your board. Usually this is 0."
 #endif
 
-#if SERIAL_PORT_2 && NUM_SERIAL < 2
+#if defined(SERIAL_PORT_2) && NUM_SERIAL < 2
   #error "SERIAL_PORT_2 is not supported for your MOTHERBOARD. Disable it to continue."
 #endif