dmaengine: tegra-apb: Remove duplicated residue calculation

The calculation of the DMA residue for the Tegra APB DMA is duplicated
in two places in the tegra_dma_tx_status() function. Remove this
duplicated code by moving calculation to the end of the function and
only calculating if we found a valid descriptor.

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