mac80211: select an AID when creating new mesh STAs

Instead of using peer link id for AID, generate a new
AID when creating mesh STAs in the kernel peering manager.
This enables smaller TIM elements and more closely follows
the standard, and it also enables mesh to work on drivers
that require a valid AID when the STA is inserted (ath10k
firmware has this requirement, for example).

In the case of userspace-managed stations, we use the AID
from NL80211_CMD_NEW_STATION.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>