Documentation: dmaengine: Correctly describe dmatest with channel unset

commit c61d7b2e upstream.

Currently the documentation states that channels must be configured before
running the dmatest. This has not been true since commit 6b41030f
("dmaengine: dmatest: Restore default for channel"). Fix accordingly.

Fixes: 6b41030f

 ("dmaengine: dmatest: Restore default for channel")
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20211118100952.27268-3-daniel.thompson@linaro.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>