ACPI: EC: PM: Flush EC work unconditionally after wakeup

Commit 607b9df6 ("ACPI: EC: PM: Avoid flushing EC work when EC
GPE is inactive") has been reported to cause some power button wakeup
events to be missed on some systems, so modify acpi_ec_dispatch_gpe()
to call acpi_ec_flush_work() unconditionally to effectively reverse
the changes made by that commit.

Also note that the problem which prompted commit 607b9df6 is not
reproducible any more on the affected machine.

Fixes: 607b9df6 ("ACPI: EC: PM: Avoid flushing EC work when EC GPE is inactive")
Reported-by: Raymond Tan <raymond.tan@intel.com>
Cc: 5.4+ <stable@vger.kernel.org> # 5.4+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>