dma: tegra20-apb-dma: Staticize tegra_dma_prep_dma_cyclic

tegra_dma_prep_dma_cyclic is referenced only in this file.
Make it static.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>