ALSA: core: init: use DECLARE_COMPLETION_ONSTACK() macro

Follow recommendation in Documentation/scheduler/completion.rst and use
macro to declare local 'struct completion'

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200902212133.30964-9-pierre-louis.bossart@linux.intel.com


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