dw_dmac: fix a regression in dwc_prep_dma_memcpy

Sometimes memory-to-memory test is failed, that's why we need to choose minimum
data portion between source and destination limits together.

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