hostap: support legacy roaming

There are currently four types of roaming, 11r roaming, 11v roaming, 11k
roaming and legacy roaming, The priority of the roaming mode is
11r-->11v-->11k-->legacy roaming. If the ap does not support 11r/11v/11k
roaming, we will use legacy roaming. legecy roaming needs to do
full channel scan, which is the same as the general scan connection
process, so the legacy roaming time will be longer.

Signed-off-by: Gaofeng Zhang <gaofeng.zhang@nxp.com>