batman-adv: use inline kernel-doc for uapi constants

The enums of constants for netlink tends to become rather large over time.
Documenting them is easier when the kernel-doc is actually next to constant
and not in a different block above the enum.

Also inline kernel-doc allows multi-paragraph description. This could be
required to better document the netlink command types and the expected
return values.

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