net/mlxe5: Separate between FEC and current speed

FEC mode is per link type, not necessary per speed. This patch access
FEC register by link modes instead of speeds. This patch will allow
further enhacment of link modes supporting FEC with the same speed
(different lane type).

Signed-off-by: Aya Levin <ayal@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>