USB: cdc-wdm: add debug messages on cleanup

Device state cleanup is done in either wdm_disconnect or
wdm_release depending on the order they are called. Adding
a couple of debug messages to document the program flow.

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