wifi: cfg80211: silence a sparse RCU warning

All we're going to do with this pointer is assign it to
another __rcu pointer, but sparse can't see that, so
use rcu_access_pointer() to silence the warning here.

Fixes: c90b93b5

 ("wifi: cfg80211: update hidden BSSes to avoid WARN_ON")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>