fdsrc: cleanup parameter initialisation and add comemnt+logging

fdsrc: cleanup parameter initialisation and add comemnt+logging

Initialize new_fd with DEFAULT_FD and fd with -1. Setting the property will set
new_fd and in _update_fd() we cehck fd against -1. Also add a coment about the
warning we get in the log from gst_poll_remove_fd(). We could get rid of the
warning if we want by tracking if fd has been added to fdset.