dmaengine: vdma: Fix compilation warning in cyclic dma mode

This patch fixes the below compilation warining.
drivers/dma/xilinx/xilinx_vdma.c: In function 'xilinx_dma_prep_dma_cyclic':
drivers/dma/xilinx/xilinx_vdma.c:1808:23: warning: 'segment' may be used
uninitialized in this function [-Wmaybe-uninitialized]
   segment->hw.control |= XILINX_DMA_BD_SOP;

The start of packet (SOP) should be set to the first segment in the desc
chain not for the last segment of the desc chain.

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>