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.

rtl837x_common.h

@@ -118,6 +118,8 @@ struct flash_region_t {
 
 extern __xdata char port_names[9][PORT_NAME_SIZE];
 
+extern __xdata uint8_t stp_enabled;
+
 /* System hostname (device identity). Set via `hostname <text>` and the System
  * Settings page, reported in /information.json. Other modules (e.g. LLDP, which
  * advertises it as the System Name TLV) read it from here. */