mac80211: disassociate prior to unlinking AP/station

This patch reorders calls during disassociation in
ieee80211_set_disassoc function.
Since sta_info_unlink calls sta_notify(REMOVE) it will
remove the station representing AP from the driver before
it has disassociated from it using bss_info_changed callback.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>