ACPI: PM: s2idle: Cancel wakeup before dispatching EC GPE

Commit 4a9af6ca ("ACPI: EC: Rework flushing of EC work while
suspended to idle") made acpi_ec_dispatch_gpe() check
pm_wakeup_pending(), but that is before canceling the SCI wakeup,
so pm_wakeup_pending() is always true.  This causes the loop in
acpi_ec_dispatch_gpe() to always terminate after one iteration which
may not be correct.

Address this issue by canceling the SCI wakeup earlier, from
acpi_ec_dispatch_gpe() itself.

Fixes: 4a9af6ca ("ACPI: EC: Rework flushing of EC work while suspended to idle")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>