dma: mmp_pdma: only complete one transaction from dma_do_tasklet()

Currently, when an interrupt has occured for a channel, the tasklet
worker code will only look at the very last entry in the running list
and complete its cookie, and then dispose the entire running chain.
Hence, the first transaction's cookie will never complete.

In fact, the interrupt we should handle will be the one related to the
first descriptor in the chain with the ENDIRQEN bit set, so complete
the second transaction that is in fact still running.

As a result, the driver can't currently handle multiple transactions on
one chanel, and it's likely that no drivers exist that rely on this
feature.

Fix this by walking the running_chain and look for the first
descriptor that has the interrupt-enable bit set. Only queue
descriptors up to that point for completion handling, while leaving
the rest intact. Also, only make the channel idle if the list is
completely empty after such a cycle.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>