tty: serial: imx: remove imx_disable_rx_int

Since imx_disable_rx_int is only called by imx_startup,
let's integrate it into that function. Notice UCR2_ATEN is
never set by the driver. The bit is still cleaned to make
this patch a noop.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Acked-by: Uwe Kleine-Koenig <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>