dma: cppi41: use cppi41_pop_desc() where possible

Use cppi41_pop_desc() when appropriate instead of open-coding the same
functionality again. That makes the code more readable. The function has
to be moved some lines up for this change.

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