OWE: Accept only BSS entries with an actual SSID for ROAM command

OWE: Accept only BSS entries with an actual SSID for ROAM command

An AP in OWE transition mode shows up in the BSS table twice due to use
of the hidden SSID. Since roaming requires the SSID to be known, don't
select the BSS entry with an empty SSID to force the one with the actual
SSID to be used when using the ROAM command to roam between APs that
might use OWE transition mode.

Fixes: 56e8f8bf3490 ("OWE: Enable roaming between OWE APs")
Signed-off-by: Jouni Malinen <j@w1.fi>