dma: xilinx: xilinx_dma: Fix compilation warning

dma: xilinx: xilinx_dma: Fix compilation warning

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:1589:43: 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: Michal Simek <michal.simek@xilinx.com>