tty: serial: imx: remove dead code imx_dma_rxint

Since commit 4dec2f11 ("imx-serial: RX DMA startup latency")
the interrupt routine no longer will start rx dma.

imx_dma_rxint no longer needs to be called to try and start dma.
It won't start dma because dma_is_rxing is
already true meaning dma is already started.

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