libertas: scan before assocation if no BSSID was given

Fix this leftover TODO from the cfg80211 conversion by doing a scan
if cfg80211 didn't pass in the BSSID for us.  Since the scan code
uses so much of the cfg80211_scan_request structure to build up the
firmware command, we just fake one when the scan request is triggered
internally.  But we need to make sure that internal 'fake' cfg82011
scan request does not get back to cfg82011 via cfg80211_scan_done().

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>