dmaengine: cppi41: Prepare to add PM runtime support

Let's just move code from cppi41_dma_issue_pending() to
push_desc_queue() as that's the only call to push_desc_queue().

We want to do this for PM runtime as we need to call push_desc_queue()
also for pending queued transfers from PM runtime resume.

No functional changes, just moves code around.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>