batman-adv: do_bcast has to be true for broadcast packets only

corrects a critical bug of the GW feature. This bug made all the unicast
packets destined to a GW to be sent as broadcast. This bug is present even if
the sender GW feature is configured as OFF. It's an urgent bug fix and should
be committed as soon as possible.

This was a regression introduced by 43676ab590c3f8686fd047d34c3e33803eef71f0

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