mxs: uart: fix setting RTS from software

With the patch "serial: mxs-auart: fix the wrong RTS hardware flow control" the
mainline mxs-uart driver now sets RTSEN only when hardware flow control is
enabled via software. It is not possible any longer to set RTS manually via
software. However, the manual modification is a valid operation.
Regain the possibility to set RTS via software and only set RTSEN when hardware
flow control is explicitly enabled via settermios cflag CRTSCTS.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Cc: stable <stable@vger.kernel.org>
Reviewed-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>