rust: dma: remove incorrect safety documentation

Removes a safety requirement that incorrectly states callers must
ensure the device does not access memory while the returned slice
is live, as this method doesn't return a slice.

Fixes: d37a39f ("rust: dma: add as_slice/write functions for CoherentAllocation")
Signed-off-by: Yilin Chen <1479826151@qq.com>
Link: https://patch.msgid.link/tencent_5195C0324923A2B67DEF8AE4B8E139BCB105@qq.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: yilin0518

rust/kernel/dma.rs

@@ -532,8 +532,6 @@ impl<T: AsBytes + FromBytes> CoherentAllocation<T> {
     ///
     /// # Safety
     ///
-    /// * Callers must ensure that the device does not read/write to/from memory while the returned
-    ///   slice is live.
     /// * Callers must ensure that this call does not race with a read or write to the same region
     ///   that overlaps with this write.
     ///