davinci-cpdma: fix locking issue in cpdma_chan_stop

Free the channel lock before calling __cpdma_chan_process to prevent
dead lock.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Tested-by: Ameya Palande <2ameya@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>