stp: take the review notes on types and register reads

stp_enabled is a flag, so say bool. cmpBytes() returns a comparison
result, so say int8_t. The three busy waits this branch adds read the
status straight out of the SFR instead of copying four bytes to xdata
first.

rtl837x_common.h

@@ -119,7 +119,7 @@ struct flash_region_t {
 
 extern __xdata char port_names[9][PORT_NAME_SIZE];
 
-extern __xdata uint8_t stp_enabled;
+extern __xdata bool 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