mac80211: fix managed mode BSSID handling

Currently, we will ask the driver to configure right away
when somebody changes the desired BSSID. That's totally
strange because then we will configure the driver without
even knowing whether the BSS exists. Change this to only
configure the BSSID when associated, and configure a zero
BSSID when not associated.

As a side effect, this fixes an issue with the iwlwifi
driver which doesn't implement sta_notify properly and
uses the BSSID instead and gets very confused if the
BSSID is cleared before we disassociate, which results
in the warning Marcel posted [1] and iwlwifi bug 1995 [2].

[1] http://thread.gmane.org/gmane.linux.kernel.wireless.general/32598
[2] http://www.intellinuxwireless.org/bugzilla/show_bug.cgi?id=1995

Cc: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>