batman-adv: fix tt_global_entries flags update

Flags carried by a change_entry have to be always copied into the
client entry as they may contain important attributes (e.g.
TT_CLIENT_WIFI).

For instance, a client added by means of the "early detection
mechanism" has no flag set at the beginning, so they must be updated once the
proper ADD event is received.

This was introduced by 30cfd02b


("batman-adv: detect not yet announced clients")

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>