usb: cdc-wdm: Fix submission of URB after suspension

There's a window under which cdc-wdm may submit
an URB to a device about to be suspended. This
introduces a flag to prevent it.

Signed-off-by: Oliver Neukum <neukum@b1-systems.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>