net/mlx5e: Fix failing ethtool query on FEC query error

If FEC caps query fails when executing 'ethtool <interface>'
the whole callback fails unnecessarily, fixed that by replacing the
error return code with debug logging only.

Fixes: 6cfa9460 ("net/mlx5e: Ethtool driver callback for query/set FEC policy")
Signed-off-by: Shay Agroskin <shayag@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>