tty: serial: imx: clear the RTSD status before enable the RTSD irq

tty: serial: imx: clear the RTSD status before enable the RTSD irq

Clear RTSD status before enabling the irq event for RTSD in
imx_uart_enable_wakeup function.
Since RTSD can be set as the wakeup source, this can avoid any risk of
false triggering of a wake-up interrupts.

Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Link: https://lore.kernel.org/r/20211125014306.4432-1-sherry.sun@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>