mlx5: implement ethtool::get_fec_stats

Report corrected bits.

v2: catch reg access errors (Saeed)

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>