batman-adv: Use 'fallthrough' pseudo keyword

The usage of the '/* fall through */' comments in switches are no longer
marked as non-deprecated variant of implicit fall throughs for switch
statements. The commit 294f69e6 ("compiler_attributes.h: Add
'fallthrough' pseudo keyword for switch/case use") introduced a replacement
keyword which should be used instead.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>