ACPI / PM / EC: Flush all EC work in acpi_freeze_sync()

Commit eed4d47e (ACPI / sleep: Ignore spurious SCI wakeups from
suspend-to-idle) introduced acpi_freeze_sync() whose purpose is to
flush all of the processing of possible wakeup events signaled via
the ACPI SCI.  However, it doesn't flush the query workqueue used
by the EC driver, so the events generated by the EC may not be
processed timely which leads to issues (increased overhead at least,
lost events possibly).

To fix that introduce acpi_ec_flush_work() that will flush all of
the outstanding EC work and call it from acpi_freeze_sync().

Fixes: eed4d47e

 (ACPI / sleep: Ignore spurious SCI wakeups from suspend-to-idle)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>