FT: Simplify FTE parsing for FT-SAE-EXT-KEY using MIC Length subfield

FT: Simplify FTE parsing for FT-SAE-EXT-KEY using MIC Length subfield

Commit 25b52e5f83f1 ("FT: Extend FTE parsing for FT-SAE-EXT-KEY") used
possible MIC length iteration to try to figure out the length of the MIC
field in FTE. That was the only option available at the time, but FTE is
now being extended in IEEE 802.11-REVme to explicitly indicate the
length of the MIC field for the new FT-SAE-EXT-KEY AKM to make this
easier.

Use the new design from the approved comment resolution (*) in
REVme/D2.0 ballot CID 3135 to simplify implementation. This gets rid of
the need to pass in key length and the somewhat strange need_{r0kh,r1kh}
parameters to wpa_ft_parse_ies().

(*)
https://mentor.ieee.org/802.11/dcn/22/11-22-1991-02-000m-proposed-resolutions-to-some-lb270-comments.docx

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