Staging: batman-adv: Don't inform about dropped packets in nodebug

Information about dropped packets are usually only interesting for
debugging purposes and otherwise open the possibility to flood the logs
of the target machine with useless information.

pr_debug will not output those information on a nodebug kernel.

Reported-by: Vasiliy Kulikov <segooon@gmail.com>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>