pcm_pulse: set prebuf parameter according to software parameters

pcm_pulse: set prebuf parameter according to software parameters

The current default value for prebuf is very high, almost the full
virtual ALSA buffer. This breaks some application especially where
low latency is involved.

This patch makes pcm_pulse implement the sw_params callback and get
the prebuf value from the ALSA software parameters. Thus the
trigger latency is much more like what an ALSA application should
expect from an ALSA PCM device.

Acked-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>