serial: imx: disable the receiver ready interrupt for imx_stop_rx

This patch disables the receiver ready interrupt for imx_stop_rx.
It reduces the interrupt numbers when the uart is going to close
or suspend.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>