mesh: Clean up AMPE element encoding and parsing

mesh: Clean up AMPE element encoding and parsing

The AMPE element includes number of optional and variable length fields
and those cannot really be represented by a fixed struct
ieee80211_ampe_ie. Remove the optional fields from the struct and
build/parse these fields separately.

This is also adding support for IGTKdata that was completely missing
from the previous implementation. In addition, Key RSC for MGTK is now
filled in and used when configuring the RX MGTK for a peer.

Signed-off-by: Jouni Malinen <j@w1.fi>