mp3_afh: Move mode_text[] into header_mode().

The array is only used in header_mode(), so it should be local to
the function.  Also, it's confusing to let the last element play the
role of the invalid header mode, and it's clearer to use ARRAY_SIZE()
instead of spelling out the index of the last element. Next, it's
unnecessary to write to the mp3header structure, so avoid this and
mark the pointer argument const.  Finally, the patch makes the array
constant, not only the mode strings.