ACPI: thinkpad-acpi: add mutex-based locking to input device event send path

Protect the input device event sending path with a mutex, since hot key
input events are not atomic and require an cohesive event block to be sent
together.

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