rust: dma: add missing __rust_helper annotations

rust: dma: add missing __rust_helper annotations

The commit d8932355f8c5 ("rust: dma: add helpers for architectures
without CONFIG_HAS_DMA") missed adding the __rust_helper annotations.
Add them.

Reported-by: Gary Guo <gary@garyguo.net>
Closes: https://lore.kernel.org/rust-for-linux/DFW4F5OSDO7A.TBUOX6RCN8G7@garyguo.net/
Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260126071738.1670967-1-dirk.behme@de.bosch.com
[ Fix minor checkpatch.pl warning. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>