dmaengine: xilinx: xdma: statify xdma_prep_interleaved_dma

dmaengine: xilinx: xdma: statify xdma_prep_interleaved_dma

xdma_prep_interleaved_dma() was local to file but not declared static,
leading to warning:

drivers/dma/xilinx/xdma.c:729:1: warning: no previous prototype for 'xdma_prep_interleaved_dma' [-Wmissing-prototypes]
  729 | xdma_prep_interleaved_dma(struct dma_chan *chan

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20231222094001.731889-1-vkoul@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>