ACPI: thinkpad-acpi: dequeue all pending hot key events at once (v2.2)

Receive all pending HKEY events at once from a single notification, and don't
complain if the queue is empty.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>