dmaengine: at_xdmac: fix potential Oops in at_xdmac_prep_interleaved()

There are two place if the at_xdmac_interleaved_queue_desc() fails which
could lead to a NULL dereference where "first" is NULL and we call
list_add_tail(&first->desc_node, ...).  In the first caller, the return
is not checked so add a check for that.  In the next caller, the return
is checked but if it fails on the first iteration through the loop then
it will lead to a NULL pointer dereference.

Fixes: 4e538578 ("dmaengine: at_xdmac: handle numf > 1")
Fixes: 62b5cb75 ("dmaengine: at_xdmac: fix memory leak in interleaved mode")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Link: https://lore.kernel.org/r/21282b66-9860-410a-83df-39c17fcf2f1b@kili.mountain


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