[PATCH] libertas: fix 'keep previous scan' behavior

Do not clear the scan list except under specific conditions, such as
when (a) user-requested, or (b) joining/starting an adhoc network.
Furthermore, only clear entries which match the SSID or BSSID of the
request, not the whole scan list.

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