USB: fix cdc-acm resume()

cdc-acm has
- a memory leak in resume()
- will fail to reactivate the read code path if this is needed.
his corrects it by deleting the useless relict code.

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