LinuxKPI: 802.11: mark struct ieee80211_ht_cap __packed

In lkpi_sta_sync_ht_from_ni() we are casting the information element (IE)
data to this struct as net80211 does not save the mcs/rx_mask.
Not marking the structs packed results in the mcs information not being
copied correctly and rx_nss calculations possibly being off. Even worse,
at a later stage iwlwifi mvm/rs.c would extract the mcs values from the
same field to pass to the firmware which will than crash in an assert
as we would want to do HT with an empty MCS set.

Sponsored by: The FreeBSD Foundation
MFC after: sunrise
Discovered: while testing other features on arm64.