modules: hostap: Supports link mode of legacy STA.

Link mode shows unknown when legacy (A or bg only) device connects to
APUT. Set the link mode to WIFI_2 when the host freq over 4000 and set
link mode to WIFI_3 when the host freq over 2000.

Signed-off-by: Hui Bai <hui.bai@nxp.com>