mac80211: restrict aggregation to supported interface modes

We can only support aggregation on AP/STA right now. HT isn't defined
for IBSS, WDS or MESH. In the WDS/MESH cases it's not clear what to
put into the IBSS field, and we don't handle that in the code at all.
Also fix the code to handle VLAN correctly.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>