libertas: rename/document scan_channel

Rename last_scanned_channel to scan_channel, just so that a
grep for struct bss_descriptor's last_scanned element doesn't
show up so many positives.

Also documented the variable and moved it to other scan related
entries in lbs_private.

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