ACPI: EC: Fix acpi_ec_dispatch_gpe()

Commit 896e97bf ("ACPI: EC: Clear GPE on interrupt handling only")
broke suspend-to-idle at least on Dell XPS13 9360 and 9380.

The problem is that acpi_ec_dispatch_gpe() must clear the EC GPE,
because the EC GPE handler never runs when the system is in the
suspend-to-idle state and if the EC GPE is not cleared by the suspend-
to-idle loop, it is never cleared at all which leads to a GPE storm.
This causes suspend-to-idle to burn energy instead of saving it which
is potentially dangerous (the affected machines heat up rather badly
when that happens).

Addess this by making acpi_ec_dispatch_gpe() clear the EC GPE as it did
before.

Fixes: 896e97bf ("ACPI: EC: Clear GPE on interrupt handling only")
Tested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>