make invalid SPI memory commands return 0xFF. fixes #1366 (Dementium II checks the results of RDSR, RDID and cmd 00 to determine what kind of backup memory is present)

src/SPI.cpp

@@ -564,6 +564,7 @@ void Write(u8 val, u32 hold)
 
     default:
         printf("unknown firmware SPI command %02X\n", CurCmd);
+        Data = 0xFF;
         break;
     }