cfg80211: show interface type

This patch makes cfg80211 show the interface in the nl80211
information about a specific interface. API users are required
to keep the type updated (everything else is fairly complicated)
but you will get a warning if you fail to keep it updated.

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