rust: dma: require mutable reference for as_slice_mut() and write()

rust: dma: require mutable reference for as_slice_mut() and write()

Given the safety requirements of as_slice_mut() and write() taking an
immutable reference is technically not incorrect.

However, let's leverage the compiler's capabilities and require a
mutable reference to ensure exclusive access.

This also fixes a clippy warning introduced with 1.88:

  warning: mutable borrow from immutable input(s)
     --> rust/kernel/dma.rs:297:78
      |
  297 |     pub unsafe fn as_slice_mut(&self, offset: usize, count: usize) -> Result<&mut [T]> {
      |                                                                              ^^^^^^^^

Fixes: d37a39f607c4 ("rust: dma: add as_slice/write functions for CoherentAllocation")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250628165120.90149-1-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>