usb: cdc-wdm: split out reusable parts of probe

Preparing for the addition of subdriver registering as an alternative
to probe for interface-less usage.  This should not change anything
apart from minor code reordering.

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