net/mlx5e: Fix setting of RS FEC mode

Change register setting from bit number to bit mask.

Fixes: b5ede32d ("net/mlx5e: Add support for FEC modes based on 50G per lane links")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Eran Ben Elisha <eranbe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>