HID: logitech-hidpp: add input_device ptr to struct hidpp_device

Most device-class specific code needs access to the input_device, instead
of storing that in the class specific data-struct, simply store this into
the hidpp_device struct itself.

In case of the m560 this avoids the need for having private data at all
and this will also avoid the need to add private data in some upcoming
patches.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>