dmaengine: pl330: Check if the DMA descriptor is NULL

Commit 6079d38c ("dmaengine: pl330: Remove useless xfer_cb indirection")
removed the __callback() function which created an unnecessary level of
indirection to execute the tranfer callback .xfer_cb

Unfortunately the commit also changed the semantics slightly since that
function used to check if the request was not NULL before attempting to
execute the callback function. Not checking this could lead to a kernel
NULL pointer dereference error.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>