dw_dmac: utilize slave_id to pass request line

There is slave_id field in the generic slave config structure that is dedicated
for the uniq slave number. In our case we have the request lines wired to the
certain hardware. Therefore the number of the request line is uniq and could be
used as slave_id. It allows us in some cases to drop out the usage of the
custom slave config structure.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>