[ALSA] cmipci: fix distortion on rear channels

When playing multichannel data, the rear channels can get distorted if
the last sample of the last played stereo stream was not zero.  To avoid
this, add a hack to play a few silence samples after the stream is
stopped.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>