thinkpad-acpi: silence hotkey enable warning for module parameter

Avoid the WARN() when the procfs handler for hotkey enable is used by
a module parameter.  Instead, urge the user to stop doing that.

Reported-by: Niel Lambrechts <niel.lambrechts@gmail.com>
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>