When there is a stream without tx_latency enabled, the fill_count ends
with MIN_FILL value. This causes one buffer of silence to be written to
every stream before the actual data in each iteration.
Consequently, more data is written than consumed in each iteration.
After several iterations, spa_bt_send fails, triggering a
group_latency_check failure in few next iterations and leading to
dropped data.
Skip streams without tx_latency enabled in fill level calculations
to prevent these audio glitches.