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.

Git-commit: d7edf40c15e85b44c4bef146819b664089b827b1
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git


Change-Id: I2ffe72be570b9c1d9569c0f7cf168407152dba95
CRs-Fixed: 2581285
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>