net/mlx5: FPGA, Call DMA unmap with the right size

When mlx5_fpga_conn_unmap_buf is called buf->sg[0].size
should equal the actual buffer size, not the message size.
Otherwise we will trigger the following dma debug warning
"DMA-API: device driver frees DMA memory with different size"

Fixes: 537a5057 ('net/mlx5: FPGA, Add high-speed connection routines')
Signed-off-by: Ilya Lesokhin <ilyal@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>