usb: gadget: composite: Initialize config->interface

Reset config->interface in usb_add_config, as it may contain pointers to
functions from a previous session if config is removed and re-added.

Signed-off-by: Benoit Goby <benoit@android.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>