ACPI: PM: s2idle: Fix comment in acpi_s2idle_prepare_late()

Fix a comment in acpi_s2idle_prepare_late() that has become outdated
after commit f0ac20c3 ("ACPI: EC: Fix flushing of pending work").

Fixes: f0ac20c3

 ("ACPI: EC: Fix flushing of pending work")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>