batman-adv: Fix kernel-doc references to struct members

The correct syntax to create references in kernel-doc to a struct member is
not "struct_name::member"" but "&struct_name->member" or
"&struct_name.member". The correct syntax is required to get the correct
cross-referencing in the reStructuredText text output.

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