mac80211: disassociate when moving to new BSS

This patch makes the MLME cleanly disassociate from the current BSS
when leaving it for a new one. This is not just nicer to the old AP
(we're leaving it, might as well tell it!) but also required for some
drivers that keep track of the station we're associated with, they'd
get confused because they'd think we are associated with two APs.

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>