flacdec: Only process a single flac audio frame.

This removes the loop in flacdec_post_select() which makes the
flac decoder decode as much as possible, but may starve other
tasks. Removing the loop gives other tasks a chance to run, thereby
avoiding buffer underruns. It simplifies the code too.

Also, resetting the number of unconsumed input bytes to zero at
each scheduler iteration has always been a mistake, so remove it.