Documentation: dmaengine: Correctly describe dmatest with channel unset

Documentation: dmaengine: Correctly describe dmatest with channel unset

Currently the documentation states that channels must be configured before
running the dmatest. This has not been true since commit 6b41030fdc79
("dmaengine: dmatest: Restore default for channel"). Fix accordingly.

Fixes: 6b41030fdc79 ("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>