dmaengine: sf-pdma: Fix an error that calls callback twice

dmaengine: sf-pdma: Fix an error that calls callback twice

Because a callback is called twice when DMA transfer complete
the second callback may be possible to access a freed memory
if the first callback routines perform the dma_release_channel function.
So this patch serialized the callback functions

Signed-off-by: Brad Kim <brad.kim@semifive.com>
Tested-and-reviewed-by: Green Wan <green.wan@sifive.com>
Signed-off-by: Brad Kim <brad.kim@sifive.com>
Link: https://lore.kernel.org/r/20200903111726.3413-1-brad.kim@sifive.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>