dmaengine: ti: k3-udma-glue: clean up k3_udma_glue_tx_get_irq() return

The k3_udma_glue_tx_get_irq() function currently returns negative error
codes on error, zero on error and positive values for success.  This
complicates life for the callers who need to propagate the error code.
Also GCC will not warn about unsigned comparisons when you check:

	if (unsigned_irq <= 0)

All the callers have been fixed now but let's just make this easy going
forward.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>