net/mlx5: remove erroneous fallthrough

This isn't a fall through because it was after a return statement.  The
fall through annotation leads to a Smatch warning:

    drivers/net/ethernet/mellanox/mlx5/core/en_ethtool.c:246
    mlx5e_ethtool_get_sset_count() warn: ignoring unreachable code.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>