dmaengine: xilinx_dma: remove set but not used variable 'tail_segment'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/dma/xilinx/xilinx_dma.c: In function 'xilinx_vdma_start_transfer':
drivers/dma/xilinx/xilinx_dma.c:1104:33: warning:
 variable 'tail_segment' set but not used [-Wunused-but-set-variable]

It not used since commit b8349172 ("dmaengine: xilinx_dma: Drop SG support
for VDMA IP")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>