iwlegacy: fix BSSID setting

Current commit 0775f9f9
"mac80211: remove spurious BSSID change flag" exposed bug on iwlegacy,
that we do not set BSSID address correctly and then device was not able
to receive frames after successful associate.

On the way fix scan canceling comment. Apparently ->post_associate()
do cancel scan itself, but scan cancel on BSS_CHANGED_BSSID is needed.
I'm not sure why, but when I removed it, I had frequent auth failures:

 wlan4: send auth to 54:e6:fc:98:63:fe (try 1/3)
 wlan4: send auth to 54:e6:fc:98:63:fe (try 2/3)
 wlan4: send auth to 54:e6:fc:98:63:fe (try 3/3)
 wlan4: authentication with 54:e6:fc:98:63:fe timed out

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>