ACPI: EC: Fix flushing of pending work

Commit 016b87ca ("ACPI: EC: Rework flushing of pending work")
introduced a subtle bug into the flushing of pending EC work while
suspended to idle, which may cause the EC driver to fail to
re-enable the EC GPE after handling a non-wakeup event (like a
battery status change event, for example).

The problem is that the work item flushed by flush_scheduled_work()
in __acpi_ec_flush_work() may disable the EC GPE and schedule another
work item expected to re-enable it, but that new work item is not
flushed, so __acpi_ec_flush_work() returns with the EC GPE disabled
and the CPU running it goes into an idle state subsequently.  If all
of the other CPUs are in idle states at that point, the EC GPE won't
be re-enabled until at least one CPU is woken up by another interrupt
source, so system wakeup events that would normally come from the EC
then don't work.

This is reproducible on a Dell XPS13 9360 in my office which
sometimes stops reacting to power button and lid events (triggered
by the EC on that machine) after switching from AC power to battery
power or vice versa while suspended to idle (each of those switches
causes the EC GPE to trigger for several times in a row, but they
are not system wakeup events).

To avoid this problem, it is necessary to drain the workqueue
entirely in __acpi_ec_flush_work(), but that cannot be done with
respect to system_wq, because work items may be added to it from
other places while __acpi_ec_flush_work() is running.  For this
reason, make the EC driver use a dedicated workqueue for EC events
processing (let that workqueue be ordered so that EC events are
processed sequentially) and use drain_workqueue() on it in
__acpi_ec_flush_work().

Fixes: 016b87ca

 ("ACPI: EC: Rework flushing of pending work")
Cc: 5.4+ <stable@vger.kernel.org> # 5.4+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>