Input: iforce - remove some pointless casts

The 'private' member of struct input_dev is a void*, so no need to
cast it when assigning it to a struct iforce* variable.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Vojtech Pavlik <vojtech@suse.cz>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>