get_prev_bmask

get prev bitmask in the enum (bitfield)
     enum_id - id of enum
     value   - value of the current bitmask
returns: value of a bitmask with value lower than the specified
         value. -1 no such bitmasks exist.
         All bitmasks are sorted by their values as unsigned longs.

long get_prev_bmask(long enum_id, long value);