dmaengine: dmatest: Restore default for channel

dmaengine: dmatest: Restore default for channel

In case of dmatest is built-in and no channel was configured test
doesn't run with:

dmatest: Could not start test, no channels configured

Even though description to "channel" parameter claims that default is
any.

Add default channel back as it used to be rather than reject test with
no channel configuration.

Fixes: d53513d5dc285d9a95a534fc41c5c08af6b60eac ("dmaengine: dmatest: Add support for multi channel testing)
Reported-by: Dijil Mohan <Dijil.Mohan@arm.com>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Link: https://lore.kernel.org/r/20200429071522.58148-1-vladimir.murzin@arm.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>