[ALSA] snd-dummy - better realtime app support

when the time interval for a period is smaller than kernel HZ, then
snd-aloop and snd-dummy cannot call snd_pcm_period_elapsed as fast enough
annymore. this happens for example with games. but the app still needs to
see, that the buffer actually did go further, which is provided by these
patches.

Signed-off-by: Ahmet İnan <ainan <at> mathematik.uni-freiburg.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>