Fix bug in serial_hexword

src/serial.c

@@ -41,7 +41,7 @@ void serial_writestring(char* s)
 const char* hexchars = "0123456789abcdef";
 void serial_hexword(uint32_t x)
 {
-    for (int i = 8; i != 0; i--) 
+    for (int i = 7; i >= 0; i--)
     {
         serial_writechar(hexchars[x>>i*4&0xf]);
     }