Some void functions have unnecessary return statements at the end
(reported by sparse) and so remove these. Also remove the return variables
from functions tegra_dma_prep_slave_sg() and tegra_dma_prep_slave_cyclic()
because the value is not used.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>