This increases the reference count on the usb cdc acm control interface
which is referred to by the tty interface provided by the driver. This
allows the deferred removal of the tty after the physical device is
disconnected if the tty is held open at the time of disconnection.

Signed-off-by: brian@murphy.dk
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>