dmaengine: ti: k3-udma: Fix the TR initialization for prep_slave_sg

dmaengine: ti: k3-udma: Fix the TR initialization for prep_slave_sg

The TR which needs to be initialized for the next sg entry is indexed by
tr_idx and not by the running i counter.

In case any sub element in the SG needs more than one TR, the code would
corrupt an already configured TR.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20200824120108.9178-1-peter.ujfalusi@ti.com
Fixes: 6cf668a4ef829 ("dmaengine: ti: k3-udma: Use the TR counter helper for slave_sg and cyclic")
Signed-off-by: Vinod Koul <vkoul@kernel.org>