Only update POST card if the POST code has actually changed

Fixes tpatx and possibly other boards taking forever to boot with the POST card enabled due to constant refreshing with the same value.

src/postcard.c

@@ -95,6 +95,9 @@ postcard_reset(void)
 static void
 postcard_write(uint16_t port, uint8_t val, void *priv)
 {
+    if (postcard_written && val == postcard_code)
+    	return;
+
     postcard_prev_code = postcard_code;
     postcard_code = val;
     if (postcard_written < 2)