batman-adv: Remove usage of BIT(x) in packet.h

The BIT(x) macro is no longer available for uapi headers because it is
defined outside of it (linux/bitops.h). The use of it must therefore be
avoided and replaced by an appropriate other representation.

Signed-off-by: Sven Eckelmann <sven.eckelmann@openmesh.com>
Signed-off-by: David S. Miller <davem@davemloft.net>