Input: xpad - fix stuck mode button on Xbox One S pad

The Xbox One S requires an ack to its mode button report, otherwise it
continuously retransmits the report. This makes the mode button appear to
be stuck down after it is pressed for the first time.

Signed-off-by: Cameron Gutman <aicommander@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>