dmaengine: xilinx_dma: Remove desc_callback_valid check

dmaengine: xilinx_dma: Remove desc_callback_valid check

In descriptor cleanup the call to desc_callback_valid can be safely
removed as both callback pointers i.e callback_result and callback
are anyway checked in invoke(). There is no much benefit in having
redundant checks.

Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: Nicholas Graumann <nick.graumann@gmail.com>
Reviewed-by: Appana Durga Kedareswara rao <appana.durga.rao@xilinx.com>
Link: https://lore.kernel.org/r/1571150904-3988-2-git-send-email-radhey.shyam.pandey@xilinx.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>