rust: cpufreq: streamline find_supply_names

Remove local variables from find_supply_names() and use .and_then() with
the more concise kernel::kvec![] macro, instead of KVec::with_capacity()
followed by .push() and Some().

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>