dma: rcar-audmapp: enable .set_slave

Current .set_slave callback did nothing,
since it assumed src/dst address come from platform settings.
But, it isn't good match to DT probing.
This patch enables .set_slave callback to this issue.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>