rtsp: don't use soon-to-be-deprecated g_cancellable_reset()

rtsp: don't use soon-to-be-deprecated g_cancellable_reset()

From the API documentation: "Note that it is generally not
a good idea to reuse an existing cancellable for more
operations after it has been cancelled once, as this
function might tempt you to do. The recommended practice
is to drop the reference to a cancellable after cancelling
it, and let it die with the outstanding async operations.
You should create a fresh cancellable for further async
operations."

https://bugzilla.gnome.org/show_bug.cgi?id=739132