net/mlx5: Fix missing error code in mlx5_init_fs()

The error code is missing in this code scenario, add the error code
'-ENOMEM' to the return value 'err'.

Eliminate the follow smatch warning:

drivers/net/ethernet/mellanox/mlx5/core/fs_core.c:2973 mlx5_init_fs()
warn: missing error code 'err'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Fixes: 4a98544d ("net/mlx5: Move chains ft pool to be used by all firmware steering").
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>