net/mlx5e: Fix a bug in turning off FEC policy in unsupported speeds

Some speeds don't support turning FEC policy off. In case a requested
FEC policy is not supported for a speed (including current speed), its new
FEC policy would be:
	no FEC - if disabling FEC is supported for that speed
	unchanged - else

Fixes: 2095b264 ("net/mlx5e: Add port FEC get/set functions")
Signed-off-by: Shay Agroskin <shayag@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>