usb: cdc-wdm: adding list lookup indirection

Register all interfaces handled by this driver in a list, getting
rid of the dependency on usb_set_intfdata.  This allows further
generalization and simplification of the probe/create functions.

This is needed to decouple wdm_open from the driver owning the
interface, and it also allows us to share all the code in
wdm_create with drivers unable to do usb_set_intfdata.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>