PM: sleep: Rename power.async_in_progress to power.work_in_progress

Rename the async_in_progress field in struct dev_pm_info to
work_in_progress as after subsequent changes it will mean work in
general rather than just async work.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/3338693.aeNJFYEL58@rjwysocki.net