Report the index of pressed touch key as MSC_SCAN code to userspace
so it is possible to identify which of the keys was pressed (not
just the function that is currently assigned to the key).

This is done similarly also in mcs_touchkey and mpr121_touchkey.
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20211013112305.41574-1-stephan@gerhold.netSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>