dmaengine: dw: extract dwc_chan_pause() for future use

iDMA 32-bit has a special handling of the FIFO during pause() /
terminate_all(). Prepare code to implement that.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>