mac80211: fils: use cfg80211_find_ext_elem()

Replace the use of cfg80211_find_ext_ie() with the more
structured cfg80211_find_ext_elem().

Link: https://lore.kernel.org/r/20210930131130.17ecf37f0605.I853c2f9c2117a713deca9b8deb3552796d98ffac@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>