dmaengine: ti: omap-dma: Readability cleanup in omap_dma_tx_status()

dmaengine: ti: omap-dma: Readability cleanup in omap_dma_tx_status()

The tx_status is most likely going to be asked for the current transfer, so
check that first then try to fall back to lookup of non started transfers.

In this way the code is a bit more readable and in most cases we will avoid
to run vchan_find_desc() all the time.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20190716082459.1222-2-peter.ujfalusi@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>