dma: cppi41: start tear down only if channel is busy

Start the channel tear down only if the channel is busy, else just
bail out. In some cases its seen that by the time the tear down is
initiated the cppi completes the DMA, especially in ISOCH transfers.

Signed-off-by: George Cherian <george.cherian@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>