hw/usb: don't set IN_ISDIR for inotify watch in MTP driver

IN_ISDIR is not a bit that one can request when registering a
watch with inotify_add_watch. Rather it is a bit that is set
automatically when reading events from the kernel.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>