dmaengine: tegra-apb: Remove unnecessary return statements and variables

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>