cdc-acm: memory leak in error case

If probe() fails not only the attributes need to be removed
but also the memory freed.

Reported-by: Ahmed Tamrawi <ahmedtamrawi@gmail.com>
Signed-off-by: Oliver Neukum <oneukum@suse.de>
CC: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>