tty: Move serial8250_stop_rx() in front of serial8250_start_tx()

Software RS485 emultaion is to be added in the following commit.
serial8250_start_tx() will need to refer serial8250_stop_rx().
Move serial8250_stop_rx() in front of serial8250_start_tx() in order
to avoid function forward declaration.

Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>