dmaengine: xgene-dma: remove unused xgene_dma_invalidate_buffer

Commit c678fa66: ("dmaengine: remove DMA_SG as it is dead code in
kernel") removes DMA_SG from dmaengine subsystem but missed removing unused
xgene_dma_invalidate_buffer function, so remove it

drivers/dma/xgene-dma.c:394:13: warning: ‘xgene_dma_invalidate_buffer’ defined but not used [-Wunused-function]
 static void xgene_dma_invalidate_buffer(__le64 *ext8)

Signed-off-by: Vinod Koul <vinod.koul@intel.com>