[media] em28xx-audio: Some Alsa API fixes

Mark the alsa stream with SNDRV_PCM_INFO_BATCH,
as the timing to get audio streams can vary.

Also, add SNDRV_PCM_TRIGGER for pause/release.

while here, fix the stop indicator, to be sure that audio
will be properly released at the stop events.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>