dmaengine: fix error codes in channel_register()

The error codes were not set on some of these error paths.

Also the error handling was more confusing than it needed to be so I
cleaned it up and shuffled it around a bit.

Fixes: d2fb0a04 ("dmaengine: break out channel registration")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20201113101631.GE168908@mwanda


Signed-off-by: Vinod Koul <vkoul@kernel.org>