dma: ti: k3-udma: Fix ring_idx to pair k3 nav rings

ring_idx was not correctly assigned in case of tflow_id is zero.
Which leads to wrong pairing of DMA for drivers like OSPI.

Fixes: 4312a1df ("dma: ti: k3-udma: Use ring_idx to pair k3 nav rings")
Reviewed-by: Jai Luthra <j-luthra@ti.com>
Signed-off-by: Udit Kumar <u-kumar1@ti.com>