dmaengine: xilinx_dma: Fix warning variable prev set but not used

This patch fixes the below sparse warning in the driver
drivers/dma/xilinx/xilinx_dma.c: In function ‘xilinx_vdma_dma_prep_interleaved’:
drivers/dma/xilinx/xilinx_dma.c:1614:43: warning: variable ‘prev’ set but not used [-Wunused-but-set-variable]
  struct xilinx_vdma_tx_segment *segment, *prev = NULL;

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