USB: cdc-acm: fix sysfs attribute registration bug

This patch (as950) fixes a bug in the cdc-acm driver.  It doesn't keep
track of which interface (control or data) the sysfs attributes get
registered for, and as a result, during disconnect it will sometimes
attempt to remove the attributes from the wrong interface.  The
left-over attributes can cause a crash later on, particularly if the driver
module has been unloaded.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Oliver Neukum <oneukum@suse.de>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>