dmaengine: dw: amend description of dma_dev field

The dma_dev field is widely used in filter functions to mach with a proper DMA
controller device. Thus it's not deprecated. The patch fixes the description of
that field. There is no functional change.

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