dmaengine: dmatest: add a maximum number of test iterations

dmaengine: dmatest: add a maximum number of test iterations

The dmatest usually waits for the killing of its kthreads to stop
running tests.  This patch adds a parameter that sets a maximum
number of test iterations.

This feature is quite interesting for debugging when you set a lot of
traces in your dmaengine controller driver.

Signed-off-by: Nicolas Ferre <[email protected]>
Cc: Haavard Skinnemoen <[email protected]>
Acked-by: Maciej Sosnowski <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Dan Williams <[email protected]>