rust: opp: Move cfg(CONFIG_OF) attribute to the top of doc test

Move the `#[cfg(CONFIG_OF)]` attribute to the top of the documentation test
block and hide it. This applies the condition to the entire test and improves
readability.

Placing configuration flags like `CONFIG_OF` at the top serves as a clear
indicator of the conditions under which the example is valid, effectively
acting like configuration metadata for the example itself.

Suggested-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://patch.msgid.link/9d93c783cc4419f16dd8942a4359d74bc0149203.1748323971.git.viresh.kumar@linaro.org
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: vireshk

rust/kernel/opp.rs

@@ -575,14 +575,14 @@ impl<T: ConfigOps + Default> Config<T> {
 /// frequency.
 ///
 /// ```
+/// # #![cfg(CONFIG_OF)]
 /// use kernel::clk::Hertz;
 /// use kernel::cpumask::Cpumask;
 /// use kernel::device::Device;
 /// use kernel::error::Result;
 /// use kernel::opp::Table;
 /// use kernel::types::ARef;
 ///
-/// #[cfg(CONFIG_OF)]
 /// fn get_table(dev: &ARef<Device>, mask: &mut Cpumask, freq: Hertz) -> Result<Table> {
 ///     let mut opp_table = Table::from_of_cpumask(dev, mask)?;
 ///