dma: imx-dma: fix imxdma_probe error path

otherwise, i will be -1 inside the latest iteration of the while loop.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>