cpuidle: fix cpuidle_find_deepest_state() kerneldoc warnings

Fix cpuidle_find_deepest_state() kernel documentation to avoid
warnings when compiling with W=1.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>