We get a compiler warn about variable ‘tail_desc’ set but not used

drivers/dma/xilinx/xilinx_dma.c:1102:42: warning:
	variable ‘tail_desc’ set but not used [-Wunused-but-set-variable]
  struct xilinx_dma_tx_descriptor *desc, *tail_desc;

So remove it.
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>