rust/mqtt: fix clippy 1.77 warning

rust/mqtt: fix clippy 1.77 warning

error: creating a mutable reference to mutable static is discouraged
   --> src/mqtt/mqtt.rs:752:23
    |
752 |     let max_msg_len = &mut MAX_MSG_LEN;
    |                       ^^^^^^^^^^^^^^^^ mutable reference to mutable static
    |
    = note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
    = note: this will be a hard error in the 2024 edition
    = note: this mutable reference has lifetime `'static`, but if the static gets accessed (read or written) by any other means, or any other reference is created, then any further use of this mutable reference is Undefined Behavior