batman-adv: Transform BATADV_LOG_BUFF(idx) into function

The linux Documentation/CodingStyle says that:
 * Chapter 12: "inline functions are preferable to macros resembling functions"
 * Chapter 12.2: Depending on local variables with a magic name is bad
 * Chapter 12.3: Macros with arguments used as l-value are bad

Signed-off-by: Sven Eckelmann <sven@narfation.org>