Disable PS2 reception when power supply is off (maybe not necessary)

This was added as I saw some strange incomming bytes when doing ctrl-alt-
delete. They were gone after this and the previous change, not sure which
commit fixed the problem.

ps2.h

@@ -438,6 +438,10 @@ class PS2KeyboardPort : public PS2Port<clkPin, datPin, size>
        Processes a scan code byte received from the keyboard
     */
     void processByteReceived(uint8_t value) {
+
+      // Power off: Do not add anything to buffer
+      if (PWR_ON_active() == false) return;
+
       // Handle BAT success (0xaa) or fail (0xfc) code
       if (!keyboardIsReady() && (value == 0xaa || value == 0xfc)) {
         bat = value;