dma: cppi41: add a delay while setting the TD bit

The manual says that we need to (repeatedly) set the TearDown-bit for
the endpoint in order to get the active transfer descriptor released.
Doing this "real" quick over and over again seems to work but it also
seems that the hardware might not have enough time to breathe. So I
though, hey lets add a udelay() between between the individual sets
of the bit.
This change with the g_zero testcase resulted in a warning about missing
transfer descriptor (we got the tear-down one). It seems that if the
hardware has some time it manages to release the transfer-descriptor on
the completion queue after the teaddown descriptor.
With this change, I observe that the transfer descriptor is released
after 20-30 retry loops.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>