cdc-wdm: untangle a circular dependency between callback and softint

We have a cycle of callbacks scheduling works which submit
URBs with those callbacks. This needs to be blocked, stopped
and unblocked to untangle the circle.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Link: https://lore.kernel.org/r/20210426092622.20433-1-oneukum@suse.com
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>