fpga: zynq_fpga: Fix the wrong usage of dma_map_sgtable()

dma_map_sgtable() returns only 0 or the error code. Read sgt->nents to
get the number of mapped segments.

Fixes: 37e0070 ("zynq_fpga: use sgtable-based scatterlist wrappers")
Reported-by: Pavel Pisa <pisa@fel.cvut.cz>
Closes: https://lore.kernel.org/linux-fpga/202508041548.22955.pisa@fel.cvut.cz/
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>
Tested-by: Pavel Pisa <pisa@fel.cvut.cz>
Link: https://lore.kernel.org/r/20250806070605.1920909-2-yilun.xu@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Xu Yilun

drivers/fpga/zynq-fpga.c

@@ -405,12 +405,12 @@ static int zynq_fpga_ops_write(struct fpga_manager *mgr, struct sg_table *sgt)
 		}
 	}
 
-	priv->dma_nelms =
-	    dma_map_sgtable(mgr->dev.parent, sgt, DMA_TO_DEVICE, 0);
-	if (priv->dma_nelms == 0) {
+	err = dma_map_sgtable(mgr->dev.parent, sgt, DMA_TO_DEVICE, 0);
+	if (err) {
 		dev_err(&mgr->dev, "Unable to DMA map (TO_DEVICE)\n");
-		return -ENOMEM;
+		return err;
 	}
+	priv->dma_nelms = sgt->nents;
 
 	/* enable clock */
 	err = clk_enable(priv->clk);