serial: pch_uart: remove unnecessary tty_port_tty_get

Remove unused tty-reference from dma-rx path which was left after the
recent tty-port conversions.

Also remove a redundant port initialisation while at it.

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