tty: serial: msm: fix potential race b/w startup and irq handling

There is a potential race for tx/rx dma requests between
msm_startup() and irq handling, for cases where dmas are not
available. This results in irq path trying to do dma mapping,
resulting in data abort. For example, consider below scenario
where rx handler reads the intermediate value of dma->chan,
set in msm_request_rx_dma(), and tries to do dma mapping,
which results in data abort.

uart_port_startup()
  msm_startup()
   request_irq()
   ...
   msm_request_rx_dma()
    ...
    dma->chan = dma_request_slave_channel_reason(dev, "rx");
    <UART RX IRQ>
     msm_uart_irq()
      msm_handle_rx_dm()
       msm_start_rx_dma()
        dma->desc = dma_map_single()
         <data abort>

Change-Id: Icf5d48f2718c3c6a855ffd3d10988a93f8281d78
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>