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

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: 37e00703228a ("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>