serial: tegra: Switch to using struct tty_port

Many of the tty functions were converted to use a struct tty_port
instead of a struct tty_struct. Update the Tegra driver accordingly to
avoid build breakage.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>