mac80211: sta_info: Add lockdep condition for RCU list usage

The function sta_info_get_by_idx() uses RCU list primitive.
It is called with  local->sta_mtx held from mac80211/cfg.c.
Add lockdep expression to avoid any false positive RCU list warnings.

Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
Link: https://lore.kernel.org/r/20200409082906.27427-1-madhuparnabhowmik10@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>