There are a few places doing the same loop iterating all PCM
substreams belonging to the PCM object.  Introduce a local helper
macro, for_each_pcm_substream(), to simplify the code.

Link: https://lore.kernel.org/r/20210206203656.15959-5-tiwai@suse.de


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