ASoC: replace use of system_wq with system_dfl_wq

ASoC: replace use of system_wq with system_dfl_wq

Currently if a user enqueue a work item using schedule_delayed_work() the
used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.

This lack of consistentcy cannot be addressed without refactoring the API.

system_wq is a per-CPU worqueue, replaced by system_percpu_wq. Despite that,
system_wq in this change has been replaced by system_dfl_wq, because there
aren't per-cpu variables.

The old wq will be kept for a few release cylces.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Link: https://patch.msgid.link/20250929155053.400342-2-marco.crivellari@suse.com
Signed-off-by: Mark Brown <broonie@kernel.org>