dmaengine: cppi41: Fix list not empty warning on module removal

If musb controller is configured with USB peripherals and we have
enumerated with a USB host, we can get warnings on removal of the
modules:

WARNING: CPU: 0 PID: 1269 at drivers/dma/cppi41.c:391
cppi41_dma_free_chan_resources

Fix the issue by adding the missing pm_runtime_get to
cppi41_dma_free_chan_resources to make sure the pending work
list is cleared on removal.

Fixes: fdea2d09

 ("dmaengine: cppi41: Add basic PM runtime support")
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>