dmaengine: xilinx: xdma: Fix some kernel-doc comments

Make the description of @xdma_chan to @xchan to silence the warnings:

drivers/dma/xilinx/xdma.c:283: warning: Function parameter or member 'xchan' not described in 'xdma_xfer_start'
drivers/dma/xilinx/xdma.c:283: warning: Excess function parameter 'xdma_chan' description in 'xdma_xfer_start'

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=4051
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Link: https://lore.kernel.org/r/20230214010344.5354-1-yang.lee@linux.alibaba.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>