batman-adv: refactor batadv_tt_local_event()

Instead of passing a generic combination of flags as
argument, it is easier to pass the entire tt_common
structure (containing the flags already set) plus a
bitfield of event flags that will be unified with
the already existing ones before inserting the client
in the event queue.
In this way invocations of the modified function can be
simplified.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>