dmaengine: dmatest: fix a small memory leak in dmatest_func()

We recently moved the test size tests around but it means we need to
adjust the error handling as well or we leak the "pq_coefs" memory.  I
updated the label name to reflect that we're freeing coefs.

Fixes: 787d3083 ("dmaengine: dmatest: move size checks earlier in function")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>