mac80211: add ieee80211_get_he_iftype_cap() helper

This function is similar to ieee80211_get_he_sta_cap() but allows passing
the iftype. Also make ieee80211_get_he_sta_cap() use the new helper
rather than duplicating the code.

Signed-off-by: Shashidhar Lakkavalli <slakkavalli@datto.com>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>