Remove use of `static mut`

[crawfxrd]

arning: creating a mutable reference to mutable static is discouraged
  --> src/../examples/util/mod.rs:54:24
   |
54 |     SpiDev::new(&mcfg, &mut LINUX_MAPPER).expect("failed to get SPI device")
   |                        ^^^^^^^^^^^^^^^^^ 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
   = note: `#[warn(static_mut_refs)]` on by default
help: use `addr_of_mut!` instead to create a raw pointer
   |
54 |     SpiDev::new(&mcfg, addr_of_mut!(LINUX_MAPPER)).expect("failed to get SPI device")
   |                        ~~~~~~~~~~~~~~~~~~~~~~~~~~