pc_serial: skip waiting for pending DPC in free hook

Since it's possible to fail queuing them, the count doesn't go down to 0,
and the process just hangs forever on exit. It might not be necessary anyway.

src/add-ons/kernel/drivers/ports/pc_serial/SerialDevice.cpp

@@ -702,8 +702,8 @@ SerialDevice::Free()
 
 	// wait until currently executing DPC is done. In case another one
 	// is run beyond this point it will just bail out on !IsOpen().
-	while (atomic_get(&fPendingDPC))
-		snooze(1000);
+	//while (atomic_get(&fPendingDPC))
+	//	snooze(1000);
 
 	gTTYModule->tty_destroy_cookie(fSystemTTYCookie);
 	gTTYModule->tty_destroy_cookie(fDeviceTTYCookie);