batman-adv: Remove one condition check in batadv_route_unicast_packet

It could decrease one condition check to collect some statements in the
first condition block.

Signed-off-by: Gao Feng <gfree.wind@gmail.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>