rust: dma: make use of start_ptr() and start_ptr_mut()

rust: dma: make use of start_ptr() and start_ptr_mut()

Using start_ptr() and start_ptr_mut() has the advantage that we inherit
the requirements the a mutable or immutable reference from those
methods.

Hence, use them instead of self.cpu_addr.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20251103190655.2326191-1-dakr@kernel.org
[ Keep using self.cpu_addr in item_from_index(). - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>