ACPI: EC: PM: Avoid flushing EC work when EC GPE is inactive

Flushing the EC work while suspended to idle when the EC GPE status
is not set causes some EC wakeup events (notably power button and
lid ones) to be missed after a series of spurious wakeups on the Dell
XPS13 9360 in my office.

If that happens, the machine cannot be woken up from suspend-to-idle
by the power button or lid status change and it needs to be woken up
in some other way (eg. by a key press).

Flushing the EC work only after successful dispatching the EC GPE,
which means that its status has been set, avoids the issue, so change
the code in question accordingly.

Fixes: 7b301750

 ("ACPI: EC: PM: Avoid premature returns from acpi_s2idle_wake()")
Cc: 5.4+ <stable@vger.kernel.org> # 5.4+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Chris Chiu <chiu@endlessm.com>