rust: follow conventions for to_* methods

`Error` is `Copy`, so no changes are required to callers

Signed-off-by: Alex Gaynor <[email protected]>

rust/kernel/error.rs

@@ -62,7 +62,7 @@ impl Error {
     }
 
     /// Returns the kernel error code.
-    pub fn to_kernel_errno(&self) -> c_types::c_int {
+    pub fn to_kernel_errno(self) -> c_types::c_int {
         self.0
     }
 }