dmaengine: dw: keep copy of custom slave config in dwc

It seems we need to extend custom slave configuration by one more member to
support Intel Quart UART. It becomes a burden to manage all members of struct
dw_dma_slave one-by-one.

Replace the set of fields by embedding struct dw_dma_slave into struct
dw_dma_chan.

Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>