dmaengine: zynqmp_dma: Fix warning variable 'val' set but not used

This patch fixes the below warning

drivers/dma/xilinx/zynqmp_dma.c: In function 'zynqmp_dma_handle_ovfl_int':
drivers/dma/xilinx/zynqmp_dma.c:522:6: warning: variable 'val' set but not used [-Wunused-but-set-variable]

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>