tty: serial: imx: disable ageing timer interrupt if dma in use

Since commit 4dec2f11 ("imx-serial: RX DMA startup latency")
the interrupt routine no longer will start rx dma.
So, we no longer need to enable this interrupt to start dma.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>