net80211: adjust more VHT structures/fields

Replace ieee80211_ie_vhtcap with ieee80211_vht_cap and
ieee80211_ie_vht_operation with ieee80211_vht_operation.
The "ie" version has the two bytes type/length at the beginning which
we did not actually use as such (the one place doing did just as unused
extra work).

Using the non-"ie" versions allows us to re-use them on shared code.
Using an enum helps us to not accidentally get unsuppored or unhandled
values tough we cannot use it in the struct as we need to ensure the
field width.

ieee80211_vht_operation is guarded by _KERNEL/WANT_NET80211. While the
header is supposed to be exported to user land historically, software
such as wpa bring their own structure definitions. For in-tree usage
it is only ifconfig which really cares (at least for now).

Sponsored by: The FreeBSD Foundation
Reviewed by: adrian (earlier), cc
Differential Revision: https://reviews.freebsd.org/D42901

(cherry picked from commit e85eb4c8d7bd8051c351a6fc6982a8b3bcfdbb2d)