ASoC: SOF: pcm: remove warning - initialize workqueue on open

If the SOF hw_params() fail, typically with an IPC error thrown by the
firmware, the period_elapsed workqueue is not initialized, but we
still cancel it in hw_free(), which results in a kernel warning.

Move the initialization to the .open callback. Tested on Broadwell
(Samus) and IceLake.

Fixes: e2803e61 ("ASoC: SOF: PCM: add period_elapsed work to fix
race condition in interrupt context")

GitHub issue: https://github.com/thesofproject/linux/issues/932


Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>