afh: Get rid of dummy entry at the end of afl[].

afh: Get rid of dummy entry at the end of afl[].

The number of audio formats is a compile-time constant. It used to be
the array size of afl[] minus one due to the dummy entry. Without
it, it becomes simply the array size. This patch introduces
NUM_AUDIO_FORMATS as a shortcut for ARRAY_SIZE(afl) and adjusts
next_audio_format() to avoid the access of memory past the end of
the array. With these preparations in place, the dummy entry can
be removed.