serial: 8250_dma: Switch to using tty_port

The tty buffer functions are converted to using tty_port
structure instead of struct tty, so we must do the same.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>