USB: cdc-acm: use get_icount tty operation

Use the tty get_icount operation instead of implementing TIOCGICOUNT
directly.

Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>