wifi: mac80211: disambiguate element parsing errors

Let the element parsing function return what kind of error
was encountered, as a bitmap, even if nothing currently
checks for which specific error it was, we'll use it later.

Link: https://msgid.link/20240129200652.1a69f2a31ec7.I55b86561d64e7ef1504c73f6f2813c33030c8136@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>