modules: hostap: flush all unused BSS entries before 11V roaming

For 11V roaming, it attempts to use previously received scan results,
if they are recent enough to be used for connection.
If wifi driver does not keep the previous scan results, it will failed
to find the bss entry.
Fix: flush all unused BSS entries, for 11V roaming, don't try to use
recent scan results.

Signed-off-by: Gang Li <gang.li_1@nxp.com>