Input: usbtouchscreen - convert from usb_device to usb_interface

Convert usbtouchscreen from storing usb_device to usb_interface. This is
needed for multi-interface touchscreen devices such as iNexio.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>