libertas: fix scan result loss if SSID IE len 0

libertas: fix scan result loss if SSID IE len 0

Scan results from Marvell 8388 and 8686 have probe responses from
hidden APs and OLPC XO-1 mesh with a zero length SSID IE.

Bug in lbs_ret_scan discarded any remaining BSS in scan response,
leading to user not seeing APs in dense environments.

With LBS_DEB_SCAN, dmesg shows

libertas scan: scan response: 5 BSSs (419 bytes); resp size 474 bytes
libertas scan: scan: 00:1a:2b:84:de:e8, capa 0401, chan  1, qz, -51 dBm
libertas scan: scan: 5c:63:bf:d8:eb:0c, capa 0411, chan  1, qw129, -23 dBm
libertas scan: scan response: invalid IE fmt

With LBS_DEB_HEX, dmesg shows valid BSS in scan response were not
processed.

Change is to ignore zero length IE and continue processing.

Fixes OLPC 12757, http://dev.laptop.org/ticket/12757

Signed-off-by: James Cameron <quozl@laptop.org>
Reported-by: T Gillett <tgillett@gmail.com>
Tested-by: T Gillett <tgillett@gmail.com>
CC: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>