Returning errno is a bug, fix that.

Also fixes smatch warnings:
drivers/net/ethernet/mellanox/mlx5/core/en/port.c:453
mlx5e_fec_in_caps() warn: signedness bug returning '(-95)'

Fixes: 2132b71f ("net/mlx5e: Advertise globaly supported FEC modes")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Aya Levin <ayal@nvidia.com>