hostap: Fix supported channels for unknown band

By default Wi-Fi works on all bands, so, the band will be unknown, in
which case no need to configure a specific frequency list to WPA
supplicant.

Only when a use provides a specific band then this is needed.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>
Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>