libertas: scan behaviour consistency improvements

When scanning for the broadcast SSID, there is no need to add the
SSID TLV (restoring the behaviour of the driver behaviour in the wext
days, confirmed in Marvell specifications).

If bssid is unspecified, the current scan code will usually fire off an
active scan probing for the specific requested SSID. However, if a scan
is ongoing (or has just finished), those scan results will be used
instead (even if that scan is totally different, e.g. a passive scan on
channel 4 for a different SSID). Fix this inconsistency by always
firing off a scan when associating without a bssid.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>