Function build_resp04 

pub fn build_resp04<const MAX_QTY: usize, R: RegisterRead>(
    out: &mut [u8],
    unit_id: u8,
    start_addr: u16,
    quantity: u16,
    regs: &R,
) -> usize

组装 FC04 响应（与 FC03 类似，只是 Func=0x04）