dmaengine: dmatest: stop completed threads when running without set channel

The completed threads were not cleared and consequent run would result
threads accumulating:

echo 800000 > /sys/module/dmatest/parameters/test_buf_size
echo 2000 > /sys/module/dmatest/parameters/timeout
echo 50 > /sys/module/dmatest/parameters/iterations
echo 1 > /sys/module/dmatest/parameters/max_channels
echo "" > /sys/module/dmatest/parameters/channel
[  237.507265] dmatest: Added 1 threads using dma1chan2
echo 1 > /sys/module/dmatest/parameters/run
[  244.713360] dmatest: Started 1 threads using dma1chan2
[  246.117680] dmatest: dma1chan2-copy0: summary 50 tests, 0 failures 2437.47 iops 977623 KB/s (0)

echo 1 > /sys/module/dmatest/parameters/run
[  292.381471] dmatest: No channels configured, continue with any
[  292.389307] dmatest: Added 1 threads using dma1chan3
[  292.394302] dmatest: Started 1 threads using dma1chan2
[  292.399454] dmatest: Started 1 threads using dma1chan3
[  293.800835] dmatest: dma1chan3-copy0: summary 50 tests, 0 failures 2624.53 iops 975014 KB/s (0)

echo 1 > /sys/module/dmatest/parameters/run
[  307.301429] dmatest: No channels configured, continue with any
[  307.309212] dmatest: Added 1 threads using dma1chan4
[  307.314197] dmatest: Started 1 threads using dma1chan2
[  307.319343] dmatest: Started 1 threads using dma1chan3
[  307.324492] dmatest: Started 1 threads using dma1chan4
[  308.730773] dmatest: dma1chan4-copy0: summary 50 tests, 0 failures 2390.28 iops 965436 KB/s (0)

Fixes: 6b41030f

 ("dmaengine: dmatest: Restore default for channel")
Reported-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20200701101225.8607-1-peter.ujfalusi@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>