dmaengine: tegra-apb: Return the actual descriptor status

dmaengine: tegra-apb: Return the actual descriptor status

Commit 71f7e6cc5500 ('dmaengine: tegra20-apb-dma: Only calculate residue
if txstate exists') changed the tegra_dma_tx_status() function to only
calculate the residue if there is a valid 'txstate' pointer for storing
the residue. Although this makes sense, this changed the behaviour of
the function tegra_dma_tx_status() such that if the pointer 'txstate' is
not valid, then we will return whatever state is returned by
dma_cookie_status() and no longer return the state by looking up the DMA
descriptor and returning it's state.

Please note that dma_cookie_status() will either return DMA_COMPLETE or
DMA_IN_PROGRESS. However, if dma_cookie_status() returns DMA_IN_PROGRESS
the actual status could be DMA_ERROR which will only be seen from
checking the descriptor status. Therefore, even if 'txstate' is not
valid, still check to see if there is a valid descriptor for the cookie
in question and if so return the descriptor state. Finally, ensure the
residue is still not calculated if the 'txstate' is not valid.

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