batman-adv: Avoid storing non-TT-sync flags on singular entries too

Since commit 54e22f26 ("batman-adv: fix TT sync flag inconsistencies")
TT sync flags and TT non-sync'd flags are supposed to be stored
separately.

The previous patch missed to apply this separation on a TT entry with
only a single TT orig entry.

This is a minor fix because with only a single TT orig entry the DDoS
issue the former patch solves does not apply.

Fixes: 54e22f26 ("batman-adv: fix TT sync flag inconsistencies")
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>