ieee80211: add helper to check HE capability element size

This element has a very dynamic structure, create a small helper
function to validate its size. We're currently checking it in
mac80211 in a conversion function, but that's actually slightly
buggy.

Link: https://lore.kernel.org/r/20220214172920.750bee9eaf37.Ie18359bd38143b7dc949078f10752413e6d36854@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>