ao_alsa: fix resuming from suspend mode

snd_pcm_prepare() was not always called, which could result in an
infinite loop.

Whether snd_pcm_prepare() was actually called depended on whether the
device was a hw device (or other characteristics; depending on
snd_pcm_hw_params_can_pause()), and required real suspend (annoying for
testing), so it was somewhat tricky to reproduce without knowing these
things.