rust: device: Use as_char_ptr() to avoid explicit cast

Use as_char_ptr() to avoid explicit cast.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/a88cd29bf01c8fbafd5c2608357f54ea10f6e492.1737016320.git.viresh.kumar@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>