wifi: mac80211: refactor adding custom elements

Rework the sorting of custom elements into the association
request by moving the elements before HT/VHT/HE to each
their own function. While at it, fix the placement of the
ones that should be between VHT and HE.

This doesn't fix the placement of elements that should be
between HE and EHT yet, a similar change might be needed
in the future.

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