dmaengine: tegra-apb: proper default init of channel slave_id

Initialize default channel slave_id(req_sel) to invalid id
(i.e max supported slave id + 1) to avoid overwriting of slave_id
during tegra_dma_slave_config() with client data if slave_id
is not initialized through DT

Signed-off-by: Shardar Shariff Md <smohammed@nvidia.com>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>