Build failure

[thejpster]

error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
   --> src/spi.rs:230:12
    |
230 |         unsafe { core::ptr::write_volatile(&self.dev.dr as *const _ as *mut u8, data) }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
    = note: even for types with interior mutability, the only legal way to obtain a mutable pointer from a shared reference is through `UnsafeCell::get`
    = note: `#[deny(invalid_reference_casting)]` on by default

error: could not compile `neotron-bmc-pico` (lib) due to 1 previous error

Seen in #72