Add missing get_icount field to two-port driver.

The two-port driver was not updated when switching to the new icount
interface in commit 0bca1b91 ("tty: Convert the USB drivers to the
new icount interface").

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>