nl80211: use for_each_element() in validate_ie_attr()

nl80211: use for_each_element() in validate_ie_attr()

This makes for much simpler code, simply walk through all
the elements and check that the last one found ends with
the end of the data. This works because if any element is
malformed the walk is aborted, we end up with a mismatch.

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