alsa: Avoid busy loop.

It is possible that snd_pcm_writei() returns zero rather than -EAGAIN
in case nothing was written because the alsa buffer was already full.

Currently we try again, and eventually succeed. However, this is
ugly, burns CPU cycles and might even lead to an endless loop for
misconfigured alsa devices. So simply return from alsa_post_select()
if snd_pcm_writei() returned zero.