Input: iforce - assign BTN_DEAD only for specific devices

Move BTN_DEAD into the arrays with the specific button sets
instead of assigning it for every iforce device.

BTN_DEAD represents an optical sensor which detects if the
user is interacting with the device. On devices without that
optical sensor BTN_DEAD is repeatedly causing false inputs
on every status update of the joystick.

Signed-off-by: Tim Schumacher <timschumi@gmx.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>