fix POWER state entry/exit based on loss of USB power (tx @mc-hamster)

src/main.cpp

@@ -128,7 +128,8 @@ class PowerFSMThread : public OSThread
 
         /// If we are in power state we force the CPU to wake every 10ms to check for serial characters (we don't yet wake
         /// cpu for serial rx - FIXME)
-        canSleep = (powerFSM.getState() != &statePOWER);
+        auto state = powerFSM.getState();
+        canSleep = (state != &statePOWER) && (state != &stateSERIAL);
 
         return 10;
     }