core: ibmpc: Add comments on stop bit of Z-150 AT

tmk_core/protocol/ibmpc.c

@@ -293,6 +293,8 @@ ISR(IBMPC_INT_VECT)
         case 0b01010000:
         case 0b11010000:
             // AT-done
+            // DO NOT check stop bit. Zenith Z-150(AT) asserts stop bit as low for no reason.
+            // https://github.com/tmk/tmk_keyboard/wiki/IBM-PC-AT-Keyboard-Protocol#zenith-z-150-beige
             // TODO: parity check?
             isr_state = isr_state>>6;
             ibmpc_protocol = IBMPC_PROTOCOL_AT;