mac80211: mlme: find auth challenge directly

mac80211: mlme: find auth challenge directly

There's no need to parse all elements etc. just to find the
authentication challenge - use cfg80211_find_elem() instead.
This also allows us to remove WLAN_EID_CHALLENGE handling
from the element parsing entirely.

Link: https://lore.kernel.org/r/20210920154009.45f9b3a15722.Ice3159ffad03a007d6154cbf1fb3a8c48489e86f@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>