dmaengine: imx-sdma: restart cyclic channel if needed

dmaengine: imx-sdma: restart cyclic channel if needed

Under heavy load resulting in high interrupt latencies, it is possible
for imx UART requests to completely fill DMA buffer. When DMA channel
is triggered and no SDMA owned buffer is available, SDMA stops. Thanks
to the autoRTS feature, there is no data loss due to the SDMA stop if
the UART is using hardware flow control.

According to DMA Engine API Guide, DMA cyclic operation is performed
until explicitly stopped. Restart the buffer after handling channel loop
if the channel was stopped by SDMA.

Signed-off-by: Tomasz Moń <tomasz.mon@camlingroup.com>
Link: https://lore.kernel.org/r/20220117091955.1038937-1-tomasz.mon@camlingroup.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>