Requests the vchan lock before using xdma->stop_request.

Fixes: 6a40fb82 ("dmaengine: xilinx: xdma: Fix synchronization issue")
Cc: stable@vger.kernel.org
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://lore.kernel.org/r/20240607-xdma-fixes-v2-1-0282319ce345@bootlin.comSigned-off-by: Vinod Koul <vkoul@kernel.org>