libertas: implement SSID scanning for SIOCSIWSCAN

After my bit scan re-writing the SIOCSIWSCAN wext ioctl no longer supported
scanning for a specific SSID. However, wpa_supplicant is a possible user of
this ioctl, so here is code that add's this.

While passing, removed even more of the debugfs-based scanning. You can (and
should) the SIOCSIWSCAN to ask for scans, so there is no need for
proprietary interfaces for scanning. And, besides, the scan result couldn't
be used further, e.g. not for associating.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>