batman-adv: Use enums for related constants

CodingStyle "Chapter 12: Macros, Enums and RTL" recommends to use enums
for several related constants. Internal states can be used without
defining the actual value, but all values which are visible to the
outside must be defined as before. Normal values are assigned as usual
and flags are defined by shifts of a bit.

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