imx serial: fix rts handling for non imx1 based hardware

The interrupt handler for muxed interrupts (imx2/imx3) was calling
the rts handling subroutine if the RTSS bit was set.
(Which indicates the status of the RTS line), leading to an
interrupt flood on RTS bit low.

This patch fixes the problem by looking at the RTSD bit instead,
indicating a change in the RTS line.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>