thinkpad_acpi: fix rfkill memory leak on unload

rfkill_unregister() should always be followed by rfkill_destroy()

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>