nl80211: modify TID-config API

Make some changes to the TID-config API:
 * use u16 in nl80211 (only, and restrict to using 8 bits for now),
   to avoid issues in the future if we ever want to use higher TIDs.
 * reject empty TIDs mask (via netlink policy)
 * change feature advertising to not use extended feature flags but
   have own mechanism for this, which simplifies the code
 * fix all variable names from 'tid' to 'tids' since it's a mask
 * change to cfg80211_ name prefixes, not ieee80211_
 * fix some minor docs/spelling things.

Change-Id: Ia234d464b3f914cdeab82f540e018855be580dce
Signed-off-by: Johannes Berg <johannes.berg@intel.com>