Make struct wpa_eapol_key easier to use with variable length MIC

Make struct wpa_eapol_key easier to use with variable length MIC

Suite B 192-bit addition from IEEE Std 802.11ac-2013 replaced the
previous fixed length Key MIC field with a variable length field. That
change was addressed with an addition of a new struct defined for the
second MIC length. This is not really scalable and with FILS coming up
with a zero-length MIC case for AEAD, a more thorough change to support
variable length MIC is needed.

Remove the Key MIC and Key Data Length fields from the struct
wpa_eapol_key and find their location based on the MIC length
information (which is determined by the AKMP). This change allows the
separate struct wpa_eapol_key_192 to be removed since struct
wpa_eapol_key will now include only the fixed length fields that are
shared with all EAPOL-Key cases in IEEE Std 802.11.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>