stp: rename stpEnabled and take the comments out of the header

The variable lives in rtlplayground.c, so it is declared in rtl837x_common.h
with the others there, and it follows the naming of the rest.

The header carried comments on the externs that the definitions in
rtl837x_stp.c repeat, sometimes differently, which is one place too many to
keep in step. What only the header had, the value ranges and what the
designated arrays hold, moved to the definitions; the rest is gone. Function
declarations lost their comments too.

The status printer only prints, so its running commentary went. A define
replaces the bare 33 in stp_in(), and the note on the loop check is down to
what applies at that line.

httpd/page_impl.c

@@ -568,7 +568,7 @@ void send_stp(void)
 	slen = strtox(outbuf, HTTP_RESPONCE_JSON);
 
 	slen += strtox(outbuf + slen, "{\"on\":");
-	bool_to_html(stpEnabled);
+	bool_to_html(stp_enabled);
 	slen += strtox(outbuf + slen, ",\"rstp\":");
 	bool_to_html(stp_rstp);
 	slen += strtox(outbuf + slen, ",\"prio\":");