dmaengine: Fix dma_async_tx_descriptor->tx_submit documentation

dmaengine: Fix dma_async_tx_descriptor->tx_submit documentation

Commit 790fb9956eea ("linux/dmaengine.h: fix a few kernel-doc
warnings") inserted new documentation for @desc_free in the middle of
@tx_submit's description.

Put @tx_submit's description back together, matching the indentation
style of the rest of the documentation for dma_async_tx_descriptor.

Fixes: 790fb9956eea ("linux/dmaengine.h: fix a few kernel-doc warnings")
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Nathan Lynch <nathan.lynch@amd.com>
Link: https://lore.kernel.org/r/20250826-dma_async_tx_desc-tx_submit-doc-fix-v1-1-18a4b51697db@amd.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>