ACPI: EC: Use a threaded handler for dedicated IRQ

After commit 7a36b901

 ("ACPI: OSL: Use a threaded interrupt handler
for SCI") all of the EC code runs in thread context on all systems where
EC events are signaled through a GPE.

It may as well run in thread context on systems using a dedicated IRQ
for EC events signaling, so make it use a threaded handler for that IRQ.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>