batman-adv: Avoid relation operator comparison with bool

commit 785ea114

 ("batman-adv: Distributed ARP Table - create DHT helper
functions") introduced a return check of batadv_compare_eth which uses a
boolean return value since commit 16af73458aca ("batman-adv: main,
batadv_compare_eth return bool"). A relational (<, >, <= or >=) operator
is not the right one for such a check.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>