net/mlx4: Fix error check for dma_map_sg

dma_map_sg return 0 on error.

Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20220825063533.21015-1-jinpu.wang@ionos.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>