Fix up serial sanity check

Marlin/src/HAL/HAL_DUE/MarlinSerial_Due.h

@@ -55,17 +55,13 @@
   #define TX_BUFFER_SIZE 32
 #endif
 
-#if ENABLED(SERIAL_XON_XOFF) && RX_BUFFER_SIZE < 1024
-  #error "XON/XOFF requires RX_BUFFER_SIZE >= 1024 for reliable transfers without drops."
-#endif
-
-#if !IS_POWER_OF_2(RX_BUFFER_SIZE) || RX_BUFFER_SIZE < 2
-  #error "RX_BUFFER_SIZE must be a power of 2 greater than 1."
-#endif
-
-#if TX_BUFFER_SIZE && (TX_BUFFER_SIZE < 2 || TX_BUFFER_SIZE > 256 || !IS_POWER_OF_2(TX_BUFFER_SIZE))
-  #error "TX_BUFFER_SIZE must be 0, a power of 2 greater than 1, and no greater than 256."
-#endif
+//#if ENABLED(SERIAL_XON_XOFF) && RX_BUFFER_SIZE < 1024
+//  #error "SERIAL_XON_XOFF requires RX_BUFFER_SIZE >= 1024 for reliable transfers without drops."
+//#elif RX_BUFFER_SIZE && (RX_BUFFER_SIZE < 2 || !IS_POWER_OF_2(RX_BUFFER_SIZE))
+//  #error "RX_BUFFER_SIZE must be a power of 2 greater than 1."
+//#elif TX_BUFFER_SIZE && (TX_BUFFER_SIZE < 2 || TX_BUFFER_SIZE > 256 || !IS_POWER_OF_2(TX_BUFFER_SIZE))
+//  #error "TX_BUFFER_SIZE must be 0, a power of 2 greater than 1, and no greater than 256."
+//#endif
 
 #if RX_BUFFER_SIZE > 256
   typedef uint16_t ring_buffer_pos_t;

Marlin/src/inc/SanityCheck.h

@@ -260,13 +260,9 @@
 #ifndef USBCON
   #if ENABLED(SERIAL_XON_XOFF) && RX_BUFFER_SIZE < 1024
     #error "SERIAL_XON_XOFF requires RX_BUFFER_SIZE >= 1024 for reliable transfers without drops."
-  #endif
-
-  #if !IS_POWER_OF_2(RX_BUFFER_SIZE) || RX_BUFFER_SIZE < 2
+  #elif RX_BUFFER_SIZE && (RX_BUFFER_SIZE < 2 || !IS_POWER_OF_2(RX_BUFFER_SIZE))
     #error "RX_BUFFER_SIZE must be a power of 2 greater than 1."
-  #endif
-
-  #if TX_BUFFER_SIZE && (TX_BUFFER_SIZE < 2 || TX_BUFFER_SIZE > 256 || !IS_POWER_OF_2(TX_BUFFER_SIZE))
+  #elif TX_BUFFER_SIZE && (TX_BUFFER_SIZE < 2 || TX_BUFFER_SIZE > 256 || !IS_POWER_OF_2(TX_BUFFER_SIZE))
     #error "TX_BUFFER_SIZE must be 0, a power of 2 greater than 1, and no greater than 256."
   #endif
 #elif ENABLED(SERIAL_XON_XOFF)