net/mlx5: Fix PPLM register mapping

Add reserved mapping to cover all the register in order to avoid
setting arbitrary values to newer FW which implements the reserved
fields.

Fixes: a58837f5

 ("net/mlx5e: Expose FEC feilds and related capability bit")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>