spa: add spa_audio_parse_position_n

Add a function that accepts the size of the position array when reading
the audio positions. This makes it possible to decouple the position
array size from SPA_AUDIO_MAX_CHANNELS.

Also use SPA_N_ELEMENTS to pass the number of array elements to
functions instead of a fixed constant. This makes it easier to change
the array size later to a different constant without having to patch up
all the places where the size is used.

src/examples/export-source.c

@@ -269,7 +269,7 @@ static int port_set_format(void *object,
 			return -EINVAL;
 		if (d->format.rate == 0 ||
 		    d->format.channels == 0 ||
-		    d->format.channels > SPA_AUDIO_MAX_CHANNELS)
+		    d->format.channels > SPA_N_ELEMENTS(d->format.position))
 			return -EINVAL;
 	}