mac80211: refactor extended element parsing

This code was really ugly, refactor it a bit to make it more
readable.

While at it, use sizeof() and fix the UORA element length
check bug.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/20200131111300.891737-4-luca@coelho.fi


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