PCI: endpoint: Replace use of system_wq with system_percpu_wq

PCI: endpoint: Replace use of system_wq with system_percpu_wq

Currently work items enqueued by schedule_delayed_work() use "system_wq" (a
per-CPU wq) while queue_delayed_work() uses 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 consistency cannot be addressed without refactoring the API.

This continues the effort to refactor workqueue APIs, which began with the
introduction of new workqueues and a new alloc_workqueue() flag in:

  128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
  930c2ea566af ("workqueue: Add new WQ_PERCPU flag")

Replace system_wq with system_percpu_wq, keeping the same behavior.  The
old wq (system_wq) will be kept for a few release cycles.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20251105151649.256274-1-marco.crivellari@suse.com