Protect the call with a mutex, as this may be called in parallel
(either from the PCM rate change and the clock change).
Acked-by: NJaroslav Kysela <perex@perex.cz>
Tested-by: NPavel Hofman <pavel.hofman@ivitera.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>