mac80211: Send Layer 2 Update frame on reassociation

When a STA roams back to the same AP before the previous STA entry has
expired, a new STA entry is not added in mac80211. However, a Layer 2
Update frame still needs to be transmitted to update layer 2 devices
about the new location for the STA. Without this, switches may
continue to forward frames to the previous (now incorrect) port when
STA roams between APs.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>