pch_uart: Fix DMA resource leak issue

pch_uart: Fix DMA resource leak issue

Changing UART mode PIO->DMA->PIO->DMA like below, pch_uart driver can't get
DMA channel resource.

setserial /dev/ttyPCH0 ^low_latency
setserial /dev/ttyPCH0 low_latency

CAUSE:
Changing mode using setserial command, ".startup" function which gets DMA
channel is called before ".verify_port" function which sets
dma-flag(use_dma/use_dma_flag) as 1.

PIO->DMA
  .startup: Since dma-flag is 0, DMA channel is not requested.
  .verify_port: dma-flag is set as 1.
  .shutdown: N/A

DMA->PIO
  .startup: Since dma-flag is 1, DMA channel is requested.
  .verify_port: dma-flag is set as 0.
  .shutdown: Since dma-flag is 0, DMA channel is not released.

This means DMA channel resource leak occurs.
Next time, this driver can't get DMA channel resource forever.

MODIFICATION:
  Currently, when release DMA channel resource, this driver checks dma-flag.
  However, this specification occurs the above issue.
  This driver must check whether dma_request_channel is executed or not.
  The values are saved in private data variable "chan_tx/chan_tx".
  These variables mean if the value is NULL, DMA channel is not requested,
  if not NULL, DMA channel is requested.

This patch fixes the issue.

Signed-off-by: Tomoya MORINAGA <tomoya.rohm@gmail.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>