pcm: dmix: Fix doubly resume of slave PCM

pcm: dmix: Fix doubly resume of slave PCM

The dmix plugin and co may trigger the resume for each instance in
snd_pcm_direct_resume().  It means that the slave PCM gets resumed or
re-prepared/started by each opened dmix stream, and this may end up
with the doubly triggers even though the slave PCM has been already
resumed by another dmix stream.

For avoiding this conflicts, check the slave PCM state and resume only
when it's still in the suspended state.  Meanwhile we keep the shadow
state updated no matter whether the slave was triggered or not.

Signed-off-by: Takashi Iwai <tiwai@suse.de>